SMTP - Simple Mail Transfer Protocol
What is SMTP?
SMTP stands for Simple Mail Transfer Protocol. It is a standard communication protocol used to send emails from one server to another over the internet.
Why is SMTP useful?
It enables email delivery between different email servers (e.g., from Gmail to Outlook).
Supports authentication and encryption to secure email transmission.
Works with other protocols like IMAP and POP3 to complete the email lifecycle (sending + receiving).
Widely supported by all major email clients and services.
How it works?
User sends an email using an email client (like Outlook or Gmail).
The email client connects to the SMTP server and sends the message.
The SMTP server processes the message and determines the recipient’s domain.
It forwards the email to the recipient’s mail server (using DNS to find it).
The recipient’s server stores the email, ready to be fetched by the user via IMAP or POP3.
Where is SMTP used?
Email clients (e.g., Outlook, Thunderbird, Apple Mail).
Mail servers (e.g., Microsoft Exchange, Postfix, Sendmail).
Webmail services (e.g., Gmail, Yahoo Mail, Zoho).
Why OSI Layer: Application Layer (Layer 7)?
It defines email commands (HELO, MAIL FROM, RCPT TO, DATA, etc.).
It interacts with user-facing applications (e.g., Outlook, Gmail).
It handles message formatting and transfer logic, not data transport or routing.
What are the key features?
Outbound Email Protocol.
Text-Based Protocol.
Supports Authentication (SMTP AUTH).
Port Flexibility.
What is an SMTP server?
A server that: * Accepts outgoing email. * Relays email to destination servers. * Examples: Sendmail, Postfix, Exim, Microsoft Exchange, Gmail SMTP.
What is a mail relay in SMTP?
A mail relay is when an SMTP server forwards email from one domain/server to another. Used in: * Email routing between domains. * Sending mail from client to provider’s SMTP.
Can SMTP be used over IPv6?
Yes. SMTP works over both IPv4 and IPv6 as long as the DNS records and mail servers support it.
What are some applications that use SMTP?
Email Clients.
Webmail Services.
Web Applications & Contact Forms.
Command-Line Tools & Scripts.
What is the working flow of SMTP?
User Composes Email * You write an email in a mail client (e.g., Outlook, Gmail, Thunderbird). * Click “Send”.
Mail Client Connects to SMTP Server * The client connects to the SMTP server on port: * 587 (with STARTTLS encryption – most common). * 465 (SSL/TLS). * 25 (for server-to-server).
Client Authenticates * The mail client logs in using: * Username and password. * Uses AUTH LOGIN or AUTH PLAIN commands.
Email Envelope is Sent * The client sends: * MAIL FROM: – sender’s email address. * RCPT TO: – recipient’s email address.
Message Content is Sent * The client sends: * DATA command. * Followed by the email body, subject, headers, and attachments (in MIME format). * Ends with a single dot . on a new line to finish the message.
Email is Queued or Relayed * The SMTP server either: * Delivers the email locally, or * Looks up recipient’s domain via DNS (MX record) and relays the message to the recipient’s mail server.
Recipient Server Accepts the Mail * The recipient mail server stores the email in the user’s inbox. * The recipient will later fetch it using POP3 or IMAP.
Client Sends QUIT * The SMTP session ends with the QUIT command. * The server closes the connection.
What are common SMTP server software options?
Postfix - Linux.
Sendmail - Linux/Unix.
Exim - Linux.
MailEnable - Windows.
hMailServer - Windows.
Topics in this section,
In this section, you are going to learn
Terminology
Version Info
S.No |
Version |
RFC |
Year |
contribution/core ideas |
|---|---|---|---|---|
1 |
SMTP |
RFC 821 |
1982 |
Original SMTP protocol for email transmission. |
2 |
ESMTP |
RFC 1869 |
1995 |
Introduced command extensions and capabilities. |
3 |
SMTP Updated |
RFC 5321 |
2008 |
Modern SMTP standard replacing RFC 2821. |
4 |
SMTP with Enhanced Status Codes |
RFC 3463 |
2003 |
Defines enhanced status codes for better diagnostics. |
5 |
SMTP Service Extensions |
RFC 4954 |
2007 |
Adds authentication (SMTP AUTH). |
6 |
SMTP over TLS |
RFC 3207 |
2002 |
Defines STARTTLS for secure SMTP communication. |
setup
setup
SMTP COMMAND LINE Packet
S.No |
Packet Components |
Description |
Size(Bytes) |
|---|---|---|---|
1 |
Command Line |
Text-based command sent by client to server |
Variable (typically < 512) |
(e.g., HELO, MAIL FROM, RCPT TO). |
|||
Command Keyword |
SMTP command like HELO, DATA, QUIT, etc. |
Variable (typically < 10) |
|
Arguments |
Parameters like domain name, email address, etc. |
Variable |
|
CRLF |
Carriage Return + Line Feed to terminate the command |
2 |
SMTP RESPONSE LINE Packet
2 |
Response Line |
Server reply to client (e.g., 250 OK, 354 Start mail input) |
Variable (typically < 512) |
|---|---|---|---|
Status Code |
3-digit numeric code indicating result (e.g., 220, 250,) |
3 |
|
Message Text |
Human-readable explanation of the status |
Variable |
|
CRLF |
Terminates the response line |
2 |
SMTP MESSAGE HEADER Packet
3 |
Message Header |
Email metadata (e.g., From, To, Subject, Date) |
Variable |
|---|---|---|---|
Header Fields |
Each field contains a key-value pair |
Variable |
|
CRLF |
Separates each header line |
2 per line |
SMTP MESSAGE BODY Packet
4 |
Message Body |
Actual content of the email message |
Variable |
|---|---|---|---|
Body Content |
Plain text or HTML content |
Variable |
|
CRLF |
Line breaks within the message |
2 per line |
SMTP END Of Data Marker Packet
S.No |
Packet Components |
Description |
Size(Bytes) |
|---|---|---|---|
5 |
End of Data Marker |
A single period (.) on a line to indicate end of message data |
3 (CRLF.) |
S.No |
Use Case |
Description |
|---|---|---|
1 |
Sending Emails |
SMTP is the standard protocol for sending emails from clients to servers. |
2 |
Server-to-Server Email Transfer |
Used to relay emails between different mail servers across the internet. |
3 |
Automated Notifications |
Applications use SMTP to send alerts, reminders, and system notifications. |
4 |
Email Marketing Campaigns |
Bulk email tools use SMTP to deliver newsletters and promotional emails. |
5 |
Contact Forms on Websites |
Websites use SMTP to send form submissions to site administrators |
6 |
Password Recovery Emails |
SMTP is used to send password reset or account recovery links. |
7 |
Two-Factor Authentication (2FA) |
Sends verification codes or login alerts to users via email. |
8 |
Application-to-User Messaging |
Apps use SMTP to send transactional emails like receipts etc. |
S.No |
STMP features |
Description |
|---|---|---|
1 |
Text-Based Protocol |
SMTP uses simple ASCII text commands for communication between |
servers. |
||
2 |
Client-Server Architecture |
Operates on a request-response model between email clients and |
servers. |
||
3 |
Store-and-Forward Mechanism |
Emails are queued and forwarded until successfully delivered. |
4 |
Port-Based Communication |
Commonly uses ports 25, 587, or 465 for sending emails. |
5 |
Command Set |
Includes commands like HELO, MAIL FROM, RCPT TO, DATA, QUIT, etc. |
6 |
Encryption Support |
Works with TLS/SSL to secure email transmission. |
7 |
Interoperability |
Compatible with all major email systems and protocols (IMAP, POP3). |
8 |
Support for Authentication |
Can use login credentials to verify sender identity (via SMTP AUTH). |
9 |
Header and Body Separation |
Clearly separates email metadata from the message content (body). |
10 |
Reliable Delivery |
Ensures messages are delivered or errors are reported back to the sender. |
Text Based Protocol - Testcases
S.No |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Connect to SMTP Server |
Attempt to connect to the SMTP server |
Connection established |
2 |
Invalid Server Address |
Use incorrect server address |
Connection fails |
3 |
Valid HELO Command |
Send valid HELO command |
Server responds with 250 OK |
4 |
Invalid HELO Syntax |
Send malformed HELO command |
Server responds with 500 Syntax error |
5 |
Valid EHLO Command |
Send valid EHLO command |
Server responds with capabilities |
6 |
MAIL FROM Valid |
Send valid MAIL FROM command |
Server responds with 250 OK |
7 |
MAIL FROM Invalid |
Send MAIL FROM with invalid syntax |
Server responds with 501 Syntax error |
8 |
RCPT TO Valid |
Send valid RCPT TO command |
Server responds with 250 OK |
9 |
RCPT TO Invalid |
Send RCPT TO with invalid address |
Server responds with 550 No such user |
10 |
DATA Command Valid |
Send DATA command after RCPT TO |
Server responds with 354 Start mail input |
11 |
DATA Command Without RCPT |
Send DATA without RCPT TO |
Server responds with 503 Bad sequence |
12 |
Message Body Format |
Send properly formatted message body |
Server accepts message |
13 |
Message Body Without End |
Send message body without terminating with “.” |
Server waits for termination |
14 |
QUIT Command |
Send QUIT command |
Server responds with 221 Bye |
15 |
AUTH LOGIN Valid |
Authenticate with valid credentials |
Server responds with 235 Authentication successful |
16 |
AUTH LOGIN Invalid |
Authenticate with invalid credentials |
Server responds with 535 Authentication failed |
17 |
AUTH PLAIN Valid |
Use AUTH PLAIN with valid credentials |
Server responds with 235 OK |
18 |
AUTH PLAIN Invalid |
Use AUTH PLAIN with invalid credentials |
Server responds with 535 Error |
19 |
STARTTLS Support |
Check if server supports STARTTLS |
Server lists STARTTLS in EHLO |
20 |
STARTTLS Initiation |
Initiate STARTTLS |
Server responds with 220 Ready to start TLS |
21 |
STARTTLS Without Support |
Send STARTTLS to server without support |
Server responds with 502 Command not implemented |
22 |
Multiple RCPT TO |
Send multiple RCPT TO commands |
Server accepts all valid recipients |
23 |
MAIL FROM After RCPT TO |
Send MAIL FROM after RCPT TO |
Server responds with 503 Bad sequence |
24 |
Invalid Command |
Send unknown command |
Server responds with 500 Unrecognized command |
25 |
Long Email Body |
Send large email body |
Server accepts and processes |
26 |
Empty MAIL FROM |
Send MAIL FROM with empty address |
Server responds with 501 Syntax error |
27 |
Empty RCPT TO |
Send RCPT TO with empty address |
Server responds with 501 Syntax error |
28 |
Multiple MAIL FROM |
Send multiple MAIL FROM commands |
Server responds with 503 Bad sequence |
29 |
Pipelining Support |
Check if server supports pipelining |
Server lists PIPELINING in EHLO |
30 |
Send Email with Attachment |
Simulate sending email with attachment |
Server accepts MIME format |
31 |
Invalid MIME Format |
Send email with malformed MIME |
Server responds with 554 Transaction failed |
32 |
Timeout on Connection |
Wait without sending data |
Server closes connection |
33 |
Reconnect After QUIT |
Connect again after QUIT |
Server allows new session |
34 |
Multiple Emails in One Session |
Send multiple emails in one session |
Server processes all correctly |
35 |
Invalid Email Address Format |
Use invalid email format |
Server responds with 501 Syntax error |
36 |
Server Banner Check |
Check server banner on connection |
Server sends 220 greeting |
37 |
VRFY Valid User |
Use VRFY with valid user |
Server responds with 250 User exists |
38 |
VRFY Invalid User |
Use VRFY with invalid user |
Server responds with 550 No such user |
39 |
EXPN Valid List |
Use EXPN with valid mailing list |
Server expands list |
40 |
EXPN Disabled |
Use EXPN when disabled |
Server responds with 502 Command not implemented |
41 |
HELP Command |
Send HELP command |
Server responds with help text |
42 |
NOOP Command |
Send NOOP command |
Server responds with 250 OK |
43 |
RSET Command |
Send RSET to reset session |
Server responds with 250 OK |
44 |
MAIL FROM with Parameters |
Use MAIL FROM with SIZE parameter |
Server accepts if within limits |
45 |
RCPT TO with Parameters |
Use RCPT TO with NOTIFY parameter |
Server accepts if supported |
46 |
Message Size Limit |
Send message exceeding size limit |
Server responds with 552 Message too large |
47 |
SMTP over SSL |
Connect using SMTPS (port 465) |
Connection secured |
48 |
SMTP Port Check |
Connect to port 25 |
Server responds if open |
49 |
SMTP Relay Test |
Attempt to relay without auth |
Server responds with 554 Relay denied |
50 |
Session Termination |
Close session abruptly |
Server logs disconnect |
Client Server Architecture - Testcases
S.No |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Server Initialization |
Start the server process |
Server starts and listens on specified port |
2 |
Client Connection |
Client attempts to connect to server |
Connection established |
3 |
Invalid Server Address |
Client uses wrong server IP/hostname |
Connection fails |
4 |
Port Availability |
Check if server port is open |
Port is accessible |
5 |
Multiple Clients |
Connect multiple clients simultaneously |
Server handles all connections |
6 |
Data Transmission |
Client sends data to server |
Server receives and processes data |
7 |
Server Response |
Server sends response to client |
Client receives correct response |
8 |
Timeout Handling |
Client waits without sending data |
Server disconnects after timeout |
9 |
Server Crash Recovery |
Simulate server crash and restart |
Server recovers and resumes service |
10 |
Unauthorized Access |
Client tries to access restricted resource |
Server denies access |
11 |
Authentication Required |
Client connects to protected service |
Server requests authentication |
12 |
Valid Authentication |
Client provides correct credentials |
Server grants access |
13 |
Invalid Authentication |
Client provides wrong credentials |
Server denies access |
14 |
Data Integrity |
Send and verify data consistency |
Data received matches sent data |
15 |
Encryption Support |
Enable encrypted communication |
Data is transmitted securely |
16 |
Protocol Compliance |
Use correct protocol (e.g., HTTP, FTP) |
Server responds appropriately |
17 |
Protocol Violation |
Use incorrect protocol format |
Server returns error |
18 |
Server Load Test |
Simulate high number of client requests |
Server remains stable |
19 |
Client Disconnection |
Client disconnects gracefully |
Server logs disconnect |
20 |
Abrupt Client Disconnect |
Client disconnects unexpectedly |
Server handles without crash |
21 |
Server Logging |
Check server logs for activity |
Logs contain accurate entries |
22 |
Client Logging |
Check client logs for communication |
Logs reflect correct data exchange |
23 |
Firewall Blocking |
Block server port via firewall |
Client cannot connect |
24 |
DNS Resolution |
Use domain name instead of IP |
Server resolves correctly |
25 |
IP Whitelisting |
Allow only specific IPs |
Unauthorized IPs are blocked |
26 |
Session Management |
Maintain session state |
Session persists across requests |
27 |
Stateless Communication |
Use stateless protocol |
Each request is independent |
28 |
Resource Access |
Client requests server resource |
Server provides correct resource |
29 |
Invalid Resource Request |
Request non-existent resource |
Server returns 404 or error |
30 |
Server Shutdown |
Gracefully shut down server |
All connections closed properly |
31 |
Server Restart |
Restart server after shutdown |
Server resumes service |
32 |
Client Retry Logic |
Retry on failed connection |
Client reconnects successfully |
33 |
Server Overload |
Exceed server capacity |
Server returns 503 or throttles |
34 |
Load Balancing |
Distribute requests across servers |
Requests handled efficiently |
35 |
Caching Mechanism |
Use server-side caching |
Cached data returned |
36 |
Session Timeout |
Inactive session expires |
Server terminates session |
37 |
Concurrent Data Access |
Multiple clients access same data |
Server handles concurrency |
38 |
Data Update Propagation |
Update data on server |
Clients receive updated data |
39 |
Version Compatibility |
Client and server versions match |
Communication succeeds |
40 |
Version Mismatch |
Client uses outdated version |
Server returns compatibility error |
41 |
API Rate Limiting |
Exceed API call limits |
Server returns rate limit error |
42 |
Secure Login |
Use HTTPS for login |
Credentials transmitted securely |
43 |
Server Configuration Change |
Modify server settings |
Server applies changes correctly |
44 |
Client Configuration Change |
Modify client settings |
Client connects successfully |
45 |
Network Latency |
Simulate high latency |
Server handles delays gracefully |
46 |
Packet Loss |
Simulate packet loss |
Server retries or reports error |
47 |
Server Monitoring |
Monitor server health |
Metrics are reported accurately |
48 |
Client Monitoring |
Monitor client activity |
Logs and metrics are available |
49 |
Error Handling |
Trigger server error |
Server returns appropriate error code |
50 |
Graceful Degradation |
Server under partial failure |
Core services remain available |
Store And forward Mechanism - Testcases
S.No |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Queue Message |
Send email when recipient server is down |
Message is queued for retry |
2 |
Retry Delivery |
Retry sending queued message |
Server retries after delay |
3 |
Retry Interval |
Check retry interval configuration |
Server waits configured time before retry |
4 |
Max Retry Limit |
Exceed maximum retry attempts |
Message is bounced |
5 |
Queue Expiry |
Message exceeds queue lifetime |
Server returns delivery failure |
6 |
Temporary Failure |
Simulate temporary DNS failure |
Message is queued |
7 |
Permanent Failure |
Simulate invalid recipient domain |
Message is rejected immediately |
8 |
Queue Monitoring |
Monitor queued messages |
Messages are visible in queue logs |
9 |
Queue Size Limit |
Exceed queue size |
Server rejects new messages |
10 |
Queue Prioritization |
Prioritize urgent messages |
High-priority messages sent first |
11 |
Message Persistence |
Server restarts during queue |
Messages remain in queue |
12 |
Queue Backup |
Backup queued messages |
Backup file created |
13 |
Queue Restore |
Restore from backup |
Messages re-queued successfully |
14 |
Message Format Integrity |
Check message format in queue |
Format remains unchanged |
15 |
Queue Encryption |
Encrypt queued messages |
Messages stored securely |
16 |
Queue Access Control |
Restrict access to queue |
Unauthorized access denied |
17 |
Queue Logging |
Log queue operations |
Logs contain enqueue/dequeue events |
18 |
Queue Flushing |
Manually flush queue |
Messages sent immediately |
19 |
Queue Inspection |
Inspect message headers |
Headers are intact |
20 |
Queue Delay Simulation |
Simulate network delay |
Message remains queued |
21 |
Queue Message Count |
Count messages in queue |
Accurate count returned |
22 |
Queue Message Size |
Check size of queued message |
Size matches original |
23 |
Queue Message Order |
Verify FIFO order |
Messages dequeued in order |
24 |
Queue Message Deletion |
Delete specific message |
Message removed from queue |
25 |
Queue Message Resend |
Resend queued message manually |
Message sent successfully |
26 |
Queue Overflow Handling |
Simulate overflow |
Server rejects new messages |
27 |
Queue Alerting |
Alert on queue threshold |
Notification triggered |
28 |
Queue Status API |
Query queue status via API |
API returns current state |
29 |
Queue Message Duplication |
Prevent duplicate messages |
No duplicates in queue |
30 |
Queue Message Compression |
Compress queued messages |
Storage optimized |
31 |
Queue Message Routing |
Route based on domain |
Correct route selected |
32 |
Queue Message TTL |
Set time-to-live for messages |
Message expires correctly |
33 |
Queue Message Retry Count |
Track retry attempts |
Count is accurate |
34 |
Queue Message Bounce |
Bounce after max retries |
Bounce message sent to sender |
35 |
Queue Message Notification |
Notify sender on delay |
Notification sent |
36 |
Queue Message Tracking |
Track delivery status |
Status updated correctly |
37 |
Queue Message Filtering |
Filter spam in queue |
Spam removed or flagged |
38 |
Queue Message Archiving |
Archive old messages |
Messages stored separately |
39 |
Queue Message Forwarding |
Forward queued message |
Message sent to alternate address |
40 |
Queue Message Validation |
Validate message before enqueue |
Invalid messages rejected |
41 |
Queue Message Retry Policy |
Apply retry policy |
Policy executed correctly |
42 |
Queue Message Retry Backoff |
Use exponential backoff |
Retry intervals increase |
43 |
Queue Message Retry Logging |
Log each retry attempt |
Logs show retry history |
44 |
Queue Message Retry Notification |
Notify on each retry |
Notification sent to admin |
45 |
Queue Message Retry Override |
Override retry settings |
New settings applied |
46 |
Queue Message Retry Success |
Retry succeeds |
Message delivered |
47 |
Queue Message Retry Failure |
Retry fails |
Message bounced |
48 |
Queue Message Retry Cancel |
Cancel retry manually |
Message removed from queue |
49 |
Queue Message Retry Resume |
Resume paused retry |
Retry continues |
50 |
Queue Message Retry Audit |
Audit retry history |
Complete audit trail available |
Port Based Communication - Testcases
S.No |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Default Port Connection |
Connect to SMTP on port 25 |
Connection established |
2 |
SMTPS Port Connection |
Connect to SMTP on port 465 |
Secure connection established |
3 |
Submission Port Connection |
Connect to SMTP on port 587 |
Authenticated connection established |
4 |
Invalid Port Number |
Connect using an invalid port |
Connection fails |
5 |
Port Blocked by Firewall |
Attempt connection with blocked port |
Connection refused |
6 |
Port Open Check |
Verify if SMTP port is open |
Port responds to connection |
7 |
Port Scan Detection |
Scan SMTP ports |
Server logs scan attempt |
8 |
Port Timeout |
Connect to port with no response |
Connection times out |
9 |
Port Redirection |
Redirect SMTP traffic to another port |
Redirection successful |
10 |
Port-Based Access Control |
Restrict access to specific ports |
Unauthorized access denied |
11 |
Port Logging |
Log all port-based connections |
Logs contain accurate entries |
12 |
Port Multiplexing |
Handle multiple services on same port |
Services respond correctly |
13 |
Port Binding Conflict |
Bind two services to same port |
Error or conflict detected |
14 |
Port-Based Load Balancing |
Distribute traffic across ports |
Load balanced successfully |
15 |
Port-Based Protocol Detection |
Detect protocol based on port |
Correct protocol identified |
16 |
Port-Based Encryption |
Use encryption on specific port |
Data transmitted securely |
17 |
Port-Based Authentication |
Require auth on specific port |
Authentication enforced |
18 |
Port-Based Rate Limiting |
Limit requests per port |
Excess requests blocked |
19 |
Port-Based Monitoring |
Monitor traffic on SMTP ports |
Metrics collected accurately |
20 |
Port-Based Alerting |
Trigger alerts on port activity |
Alerts sent on threshold breach |
21 |
Port-Based Throttling |
Throttle traffic on busy port |
Traffic slowed appropriately |
22 |
Port-Based Session Handling |
Manage sessions per port |
Sessions tracked correctly |
23 |
Port-Based Protocol Switching |
Switch protocols based on port |
Correct protocol used |
24 |
Port-Based NAT Traversal |
Handle NAT for SMTP ports |
Communication succeeds |
25 |
Port-Based QoS |
Apply Quality of Service rules |
Priority traffic handled first |
26 |
Port-Based Firewall Rules |
Apply rules per port |
Rules enforced correctly |
27 |
Port-Based IDS/IPS |
Detect intrusions on SMTP ports |
Threats identified and blocked |
28 |
Port-Based Logging Format |
Verify log format for port activity |
Logs are structured correctly |
29 |
Port-Based Connection Retry |
Retry failed port connection |
Connection reattempted |
30 |
Port-Based Connection Drop |
Drop connection on specific port |
Connection terminated |
31 |
Port-Based Connection Persistence |
Maintain persistent connection |
Connection remains active |
32 |
Port-Based Connection Termination |
Terminate connection gracefully |
Server sends termination response |
33 |
Port-Based Connection Reuse |
Reuse existing connection |
Connection reused successfully |
34 |
Port-Based Connection Pooling |
Pool connections per port |
Pool managed efficiently |
35 |
Port-Based Connection Isolation |
Isolate traffic per port |
No cross-port interference |
36 |
Port-Based Connection Encryption |
Encrypt traffic per port |
Data secured |
37 |
Port-Based Connection Compression |
Compress traffic per port |
Data size reduced |
38 |
Port-Based Connection Validation |
Validate connection parameters |
Parameters accepted |
39 |
Port-Based Connection Filtering |
Filter traffic per port |
Unwanted traffic blocked |
40 |
Port-Based Connection Statistics |
Collect stats per port |
Accurate metrics reported |
41 |
Port-Based Connection Debugging |
Debug issues per port |
Logs and traces available |
42 |
Port-Based Connection Testing |
Test connection health |
Connection verified |
43 |
Port-Based Connection Failover |
Switch to backup port |
Failover successful |
44 |
Port-Based Connection Recovery |
Recover from port failure |
Connection restored |
45 |
Port-Based Connection Audit |
Audit port usage |
Detailed report generated |
46 |
Port-Based Connection Security |
Apply security policies |
Policies enforced |
47 |
Port-Based Connection Optimization |
Optimize traffic per port |
Performance improved |
48 |
Port-Based Connection Configuration |
Configure port settings |
Settings applied correctly |
49 |
Port-Based Connection Synchronization |
Sync data across ports |
Data synchronized |
50 |
Port-Based Connection Verification |
Verify connection integrity |
Connection verified successfully |
Command Set Feature - Testcases
S.No |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
HELO Command |
Send valid HELO command |
Server responds with 250 OK |
2 |
EHLO Command |
Send valid EHLO command |
Server responds with capabilities |
3 |
MAIL FROM Command |
Send valid MAIL FROM command |
Server responds with 250 OK |
4 |
RCPT TO Command |
Send valid RCPT TO command |
Server responds with 250 OK |
5 |
DATA Command |
Send DATA command after RCPT TO |
Server responds with 354 Start mail input |
6 |
QUIT Command |
Send QUIT command |
Server responds with 221 Bye |
7 |
RSET Command |
Send RSET to reset session |
Server responds with 250 OK |
8 |
NOOP Command |
Send NOOP command |
Server responds with 250 OK |
9 |
VRFY Command |
Verify a valid user |
Server responds with 250 User exists |
10 |
EXPN Command |
Expand a mailing list |
Server responds with list or 502 if disabled |
11 |
HELP Command |
Request help information |
Server responds with help text |
12 |
AUTH LOGIN |
Authenticate using LOGIN method |
Server responds with 235 OK |
13 |
AUTH PLAIN |
Authenticate using PLAIN method |
Server responds with 235 OK |
14 |
AUTH CRAM-MD5 |
Authenticate using CRAM-MD5 |
Server responds with 235 OK |
15 |
STARTTLS Command |
Initiate TLS encryption |
Server responds with 220 Ready to start TLS |
16 |
Invalid HELO Syntax |
Send malformed HELO |
Server responds with 500 Syntax error |
17 |
Invalid MAIL FROM |
Send MAIL FROM with bad format |
Server responds with 501 Syntax error |
18 |
Invalid RCPT TO |
Send RCPT TO with bad format |
Server responds with 501 Syntax error |
19 |
DATA Without RCPT TO |
Send DATA before RCPT TO |
Server responds with 503 Bad sequence |
20 |
Multiple RCPT TO |
Send multiple RCPT TO commands |
Server accepts all valid recipients |
21 |
MAIL FROM After RCPT TO |
Send MAIL FROM after RCPT TO |
Server responds with 503 Bad sequence |
22 |
Unknown Command |
Send an undefined command |
Server responds with 500 Unrecognized command |
23 |
AUTH Without EHLO |
Send AUTH before EHLO |
Server responds with 503 Bad sequence |
24 |
STARTTLS Without EHLO |
Send STARTTLS before EHLO |
Server responds with 503 Bad sequence |
25 |
AUTH With Invalid Credentials |
Send wrong credentials |
Server responds with 535 Authentication failed |
26 |
AUTH With Valid Credentials |
Send correct credentials |
Server responds with 235 Authentication successful |
27 |
STARTTLS Repeated |
Send STARTTLS twice |
Server responds with 503 Already encrypted |
28 |
DATA With No End |
Send DATA without terminating with “.” |
Server waits for termination |
29 |
DATA With Invalid Format |
Send malformed message body |
Server responds with 554 Transaction failed |
30 |
VRFY Disabled |
Use VRFY when disabled |
Server responds with 502 Command not implemented |
31 |
EXPN Disabled |
Use EXPN when disabled |
Server responds with 502 Command not implemented |
32 |
HELP With Argument |
Send HELP with specific command |
Server responds with command help |
33 |
NOOP Multiple Times |
Send NOOP repeatedly |
Server responds with 250 OK each time |
34 |
RSET After DATA |
Send RSET after DATA command |
Server resets session |
35 |
MAIL FROM With Parameters |
Use MAIL FROM with SIZE param |
Server accepts if within limits |
36 |
RCPT TO With Parameters |
Use RCPT TO with NOTIFY param |
Server accepts if supported |
37 |
AUTH With Empty Credentials |
Send empty credentials |
Server responds with 535 Authentication failed |
38 |
AUTH With Base64 Encoding |
Send credentials in Base64 |
Server decodes and authenticates |
39 |
AUTH With Incorrect Encoding |
Send malformed Base64 |
Server responds with 501 Syntax error |
40 |
STARTTLS With Invalid Certificate |
Use invalid TLS cert |
Connection fails |
41 |
STARTTLS With Valid Certificate |
Use valid TLS cert |
Connection secured |
42 |
EHLO With Domain |
Send EHLO with domain name |
Server responds with 250 and capabilities |
43 |
EHLO With IP Address |
Send EHLO with IP |
Server responds with 250 OK |
44 |
MAIL FROM With Empty Address |
Send MAIL FROM:<> |
Server accepts as bounce |
45 |
RCPT TO With Empty Address |
Send RCPT TO:<> |
Server responds with 501 Syntax error |
46 |
DATA With MIME Format |
Send MIME formatted message |
Server accepts message |
47 |
DATA With HTML Content |
Send HTML email body |
Server accepts message |
48 |
QUIT After AUTH |
Send QUIT after authentication |
Server ends session |
49 |
QUIT Without AUTH |
Send QUIT without authentication |
Server ends session |
50 |
AUTH With Multiple Methods |
Try multiple AUTH methods |
Server selects supported method |
Encryption Support - Testcases
S.No |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
STARTTLS Command |
Send STARTTLS command |
Server responds with 220 Ready to start TLS |
2 |
STARTTLS Support Check |
Check if server supports STARTTLS |
Listed in EHLO response |
3 |
TLS Handshake Success |
Complete TLS handshake |
Secure connection established |
4 |
TLS Handshake Failure |
Simulate handshake failure |
Connection terminated |
5 |
SSL Port Connection |
Connect to SMTPS on port 465 |
Encrypted connection established |
6 |
STARTTLS Without EHLO |
Send STARTTLS before EHLO |
Server responds with 503 Bad sequence |
7 |
STARTTLS Repeated |
Send STARTTLS twice |
Server responds with 503 Already encrypted |
8 |
STARTTLS With Invalid Certificate |
Use invalid TLS certificate |
Connection fails |
9 |
STARTTLS With Valid Certificate |
Use valid TLS certificate |
Connection secured |
10 |
Encrypted MAIL FROM |
Send MAIL FROM over TLS |
Server accepts securely |
11 |
Encrypted RCPT TO |
Send RCPT TO over TLS |
Server accepts securely |
12 |
Encrypted DATA |
Send email body over TLS |
Server accepts securely |
13 |
Encrypted AUTH LOGIN |
Authenticate over TLS |
Credentials transmitted securely |
14 |
Encrypted AUTH PLAIN |
Use AUTH PLAIN over TLS |
Credentials transmitted securely |
15 |
Encrypted AUTH CRAM-MD5 |
Use CRAM-MD5 over TLS |
Authentication succeeds |
16 |
TLS Version Support |
Check supported TLS versions |
Server lists supported versions |
17 |
TLSv1.2 Connection |
Connect using TLSv1.2 |
Connection succeeds |
18 |
TLSv1.3 Connection |
Connect using TLSv1.3 |
Connection succeeds |
19 |
SSLv3 Connection Attempt |
Attempt SSLv3 connection |
Server rejects insecure protocol |
20 |
Cipher Suite Negotiation |
Negotiate cipher suite |
Strong cipher selected |
21 |
Weak Cipher Rejection |
Attempt connection with weak cipher |
Server rejects connection |
22 |
Certificate Expiry Check |
Use expired certificate |
Connection fails |
23 |
Certificate Validity Check |
Use valid certificate |
Connection succeeds |
24 |
Certificate Revocation Check |
Use revoked certificate |
Connection fails |
25 |
Certificate Chain Validation |
Validate full chain |
Server accepts if valid |
26 |
Self-Signed Certificate |
Use self-signed cert |
Connection may be rejected |
27 |
CA-Signed Certificate |
Use CA-signed cert |
Connection accepted |
28 |
Encrypted Session Persistence |
Maintain encrypted session |
Session remains secure |
29 |
Encrypted Session Termination |
Terminate encrypted session |
Server closes securely |
30 |
Encrypted Session Reuse |
Reuse TLS session |
Server supports reuse |
31 |
TLS Downgrade Attack Prevention |
Attempt downgrade |
Server prevents downgrade |
32 |
TLS Renegotiation |
Attempt renegotiation |
Server handles securely |
33 |
TLS Alert Handling |
Trigger TLS alert |
Server responds appropriately |
34 |
TLS Logging |
Log TLS session details |
Logs contain handshake info |
35 |
TLS Metrics Collection |
Collect encryption metrics |
Metrics reported accurately |
36 |
TLS Error Reporting |
Report TLS errors |
Server logs and returns error |
37 |
TLS Performance Test |
Measure encrypted session speed |
Acceptable performance |
38 |
TLS Compatibility Test |
Test across clients |
All supported clients connect securely |
39 |
TLS Certificate Rotation |
Rotate server certificate |
New cert accepted |
40 |
TLS Certificate Pinning |
Use pinned certificate |
Connection succeeds only with match |
41 |
TLS Session Timeout |
Let session expire |
Server terminates session |
42 |
TLS Session Resume |
Resume session after timeout |
Server resumes securely |
43 |
TLS Session Cache |
Cache session info |
Server uses cache correctly |
44 |
TLS Session ID Check |
Verify session ID |
ID matches expected value |
45 |
TLS Session Ticket Check |
Verify session ticket |
Ticket used correctly |
46 |
TLS Client Certificate |
Use client cert for auth |
Server validates certificate |
47 |
TLS Mutual Authentication |
Require client and server certs |
Mutual auth succeeds |
48 |
TLS Certificate Fingerprint |
Verify fingerprint |
Matches expected value |
49 |
TLS Certificate Subject Check |
Check subject fields |
Fields match expected values |
50 |
TLS Certificate SAN Check |
Check Subject Alternative Name |
SAN includes correct domains |
interoperability - Testcases
S.No |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
SMTP Client Compatibility |
Test with different SMTP clients |
All clients connect successfully |
2 |
SMTP Server Compatibility |
Test with different SMTP servers |
Communication succeeds |
3 |
Cross-Platform Support |
Test on Windows, Linux, macOS |
SMTP works consistently |
4 |
Email Format Compatibility |
Send plain text and HTML emails |
All formats delivered correctly |
5 |
MIME Type Handling |
Send emails with various MIME types |
MIME types processed correctly |
6 |
Attachment Compatibility |
Send attachments from different clients |
Attachments received intact |
7 |
Character Encoding Support |
Send emails with UTF-8, ASCII, etc. |
Encoding preserved |
8 |
Language Support |
Send emails in multiple languages |
Content displayed correctly |
9 |
TLS Version Compatibility |
Test TLS 1.2, 1.3 |
Secure connection established |
10 |
Cipher Suite Compatibility |
Use various cipher suites |
Negotiation succeeds |
11 |
Authentication Method Compatibility |
Test LOGIN, PLAIN, CRAM-MD5 |
All supported methods work |
12 |
IPv4 and IPv6 Support |
Connect using both IP versions |
Connection succeeds |
13 |
DNS Resolution |
Resolve domain names across platforms |
Resolution successful |
14 |
SMTP Relay Compatibility |
Relay through different servers |
Message forwarded correctly |
15 |
Port Compatibility |
Use ports 25, 465, 587 |
All ports function correctly |
16 |
Email Client Rendering |
Check rendering in Outlook, Gmail, etc. |
Email displays correctly |
17 |
Header Field Compatibility |
Use standard headers |
Headers interpreted correctly |
18 |
Custom Header Support |
Use custom headers |
Headers preserved |
19 |
BCC and CC Compatibility |
Send with BCC and CC |
Recipients handled correctly |
20 |
Time Zone Handling |
Send emails across time zones |
Timestamps accurate |
21 |
Date Format Compatibility |
Use different date formats |
Server parses correctly |
22 |
HTML Email Rendering |
Send HTML content |
Rendered properly in clients |
23 |
Inline Image Support |
Send inline images |
Displayed correctly |
24 |
Email Threading |
Maintain conversation threads |
Threading preserved |
25 |
Delivery Status Notification |
Request DSN |
Notification received |
26 |
Read Receipt Compatibility |
Request read receipt |
Receipt received if supported |
27 |
SMTP Extension Support |
Use supported extensions |
Server responds correctly |
28 |
STARTTLS Compatibility |
Use STARTTLS across clients |
Secure connection established |
29 |
Email Forwarding |
Forward email across domains |
Message forwarded correctly |
30 |
Email Filtering |
Apply filters on received emails |
Filters work correctly |
31 |
Spam Detection Compatibility |
Test spam filters |
Spam flagged appropriately |
32 |
DKIM Signature Verification |
Verify DKIM signature |
Signature validated |
33 |
SPF Record Check |
Validate SPF records |
SPF check passes |
34 |
DMARC Policy Enforcement |
Apply DMARC policy |
Policy enforced correctly |
35 |
SMTP over VPN |
Send email via VPN |
Message delivered securely |
36 |
SMTP over Proxy |
Send email via proxy |
Message delivered correctly |
37 |
SMTP over Mobile Network |
Send email via mobile data |
Message delivered successfully |
38 |
SMTP over Satellite |
Send email via satellite link |
Message delivered with delay |
39 |
SMTP over Cloud Services |
Use cloud SMTP services |
Message delivered correctly |
40 |
SMTP API Integration |
Use SMTP via API |
API sends email successfully |
41 |
SMTP with Webmail |
Send email via webmail |
Message delivered correctly |
42 |
SMTP with Desktop Client |
Send email via desktop client |
Message delivered correctly |
43 |
SMTP with Mobile App |
Send email via mobile app |
Message delivered correctly |
44 |
SMTP with IoT Device |
Send email from IoT device |
Message delivered correctly |
45 |
SMTP with CRM System |
Send email from CRM |
Message delivered correctly |
46 |
SMTP with ERP System |
Send email from ERP |
Message delivered correctly |
47 |
SMTP with Ticketing System |
Send email from ticketing tool |
Message delivered correctly |
48 |
SMTP with Monitoring Tool |
Send alerts via SMTP |
Alerts delivered correctly |
49 |
SMTP with Automation Tool |
Send email via automation |
Message delivered correctly |
50 |
SMTP with Backup System |
Send logs via SMTP |
Logs delivered correctly |
Support For Authentication - Testcases
S.No |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
AUTH LOGIN Command |
Send AUTH LOGIN command |
Server prompts for credentials |
2 |
AUTH PLAIN Command |
Send AUTH PLAIN command |
Server accepts credentials |
3 |
AUTH CRAM-MD5 Command |
Send AUTH CRAM-MD5 command |
Server performs challenge-response |
4 |
AUTH Without EHLO |
Send AUTH before EHLO |
Server responds with 503 Bad sequence |
5 |
AUTH With Valid Credentials |
Provide correct username and password |
Server responds with 235 Authentication successful |
6 |
AUTH With Invalid Credentials |
Provide incorrect credentials |
Server responds with 535 Authentication failed |
7 |
AUTH With Empty Credentials |
Send empty username/password |
Server responds with 535 Authentication failed |
8 |
AUTH With Base64 Encoding |
Send credentials in Base64 |
Server decodes and authenticates |
9 |
AUTH With Malformed Encoding |
Send malformed Base64 |
Server responds with 501 Syntax error |
10 |
AUTH LOGIN Step-by-Step |
Send username and password separately |
Server authenticates successfully |
11 |
AUTH PLAIN Inline |
Send credentials inline |
Server authenticates successfully |
12 |
AUTH CRAM-MD5 Challenge |
Respond to server challenge |
Server validates response |
13 |
AUTH Method Listing |
Check EHLO response for supported methods |
Server lists available AUTH methods |
14 |
AUTH Method Not Supported |
Use unsupported method |
Server responds with 504 Unrecognized authentication type |
15 |
AUTH Over STARTTLS |
Authenticate after STARTTLS |
Credentials transmitted securely |
16 |
AUTH Without STARTTLS |
Authenticate without encryption |
Server may reject or warn |
17 |
AUTH With Expired Account |
Use expired credentials |
Server responds with 535 Authentication failed |
18 |
AUTH With Locked Account |
Use locked credentials |
Server responds with 535 Authentication failed |
19 |
AUTH With Disabled Method |
Use disabled AUTH method |
Server responds with 504 Error |
20 |
AUTH With Long Username |
Use excessively long username |
Server responds with 501 or 535 error |
21 |
AUTH With Long Password |
Use excessively long password |
Server responds with 501 or 535 error |
22 |
AUTH With Special Characters |
Use special characters in credentials |
Server handles correctly |
23 |
AUTH With Unicode Characters |
Use Unicode in credentials |
Server handles correctly |
24 |
AUTH Retry After Failure |
Retry after failed attempt |
Server allows retry |
25 |
AUTH Retry Limit |
Exceed retry limit |
Server blocks further attempts |
26 |
AUTH Logging |
Check server logs for AUTH attempts |
Logs contain accurate entries |
27 |
AUTH Audit Trail |
Verify audit trail of authentication |
Trail is complete and secure |
28 |
AUTH Session Persistence |
Maintain session after authentication |
Session remains active |
29 |
AUTH Session Timeout |
Let session expire |
Server requires re-authentication |
30 |
AUTH Session Termination |
Terminate session manually |
Server ends session |
31 |
AUTH With Multiple Users |
Authenticate different users |
Server handles all correctly |
32 |
AUTH With Role-Based Access |
Authenticate with roles |
Access granted based on role |
33 |
AUTH With IP Restrictions |
Authenticate from restricted IP |
Server denies access |
34 |
AUTH With Rate Limiting |
Exceed login attempts per minute |
Server throttles requests |
35 |
AUTH With CAPTCHA |
Use CAPTCHA in web-based SMTP |
CAPTCHA validated |
36 |
AUTH With MFA |
Use multi-factor authentication |
Server validates second factor |
37 |
AUTH With Token |
Use access token for authentication |
Server validates token |
38 |
AUTH With OAuth |
Authenticate via OAuth |
Server redirects and validates |
39 |
AUTH With LDAP |
Authenticate via LDAP backend |
Server validates credentials |
40 |
AUTH With Active Directory |
Authenticate via AD |
Server validates credentials |
41 |
AUTH With Database |
Authenticate via DB lookup |
Server validates credentials |
42 |
AUTH With External API |
Authenticate via external service |
Server validates via API |
43 |
AUTH With Failover |
Failover to backup auth service |
Authentication succeeds |
44 |
AUTH With Logging Disabled |
Disable logging for privacy |
No logs generated |
45 |
AUTH With Logging Enabled |
Enable detailed logging |
Logs contain full trace |
46 |
AUTH With Encryption |
Encrypt credentials in transit |
Data secured |
47 |
AUTH With Replay Attack |
Attempt replay of credentials |
Server detects and blocks |
48 |
AUTH With Brute Force |
Simulate brute force attack |
Server blocks IP or user |
49 |
AUTH With Session Hijack |
Attempt session hijack |
Server detects and terminates |
50 |
AUTH With Audit Compliance |
Check compliance with standards |
Authentication meets policy requirements |
Header And Body Seperation - Testcases
S.No |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Valid Header and Body |
Send email with proper header-body separation |
Server accepts message |
2 |
Missing Header |
Send email with only body |
Server rejects or flags message |
3 |
Missing Body |
Send email with only headers |
Server accepts but message is empty |
4 |
No Separation Line |
Omit blank line between header and body |
Server rejects or misinterprets message |
5 |
Multiple Blank Lines |
Use multiple blank lines between header and body |
Server processes correctly |
6 |
Header Field Order |
Send headers in standard order |
Server accepts message |
7 |
Header Field Disorder |
Send headers in random order |
Server accepts if fields are valid |
8 |
Invalid Header Format |
Use malformed header fields |
Server rejects or flags message |
9 |
Duplicate Header Fields |
Repeat header fields like Subject |
Server handles or flags duplicates |
10 |
Case Sensitivity in Headers |
Use mixed case in header names |
Server processes correctly |
11 |
Custom Header Fields |
Include custom headers |
Server preserves them |
12 |
Long Header Lines |
Use long header lines |
Server wraps or accepts based on RFC |
13 |
Encoded Header Fields |
Use encoded values in headers |
Server decodes correctly |
14 |
MIME Version Header |
Include MIME-Version header |
Server accepts MIME format |
15 |
Content-Type Header |
Specify content type |
Server interprets correctly |
16 |
Content-Transfer-Encoding |
Specify encoding method |
Server processes accordingly |
17 |
Date Header Format |
Use correct date format |
Server accepts message |
18 |
Invalid Date Header |
Use malformed date |
Server flags or rejects message |
19 |
From Header Valid |
Use valid sender address |
Server accepts message |
20 |
From Header Invalid |
Use malformed sender address |
Server rejects message |
21 |
To Header Valid |
Use valid recipient address |
Server accepts message |
22 |
To Header Invalid |
Use malformed recipient address |
Server rejects message |
23 |
Subject Header Present |
Include subject line |
Server accepts message |
24 |
Subject Header Missing |
Omit subject line |
Server accepts but may flag message |
25 |
Body with Plain Text |
Send plain text body |
Server accepts message |
26 |
Body with HTML Content |
Send HTML body |
Server accepts and delivers correctly |
27 |
Body with Mixed Content |
Send multipart body |
Server processes MIME correctly |
28 |
Body with Inline Images |
Include inline images |
Server accepts if MIME is correct |
29 |
Body with Attachments |
Include attachments in body |
Server accepts if formatted correctly |
30 |
Body with Special Characters |
Include special characters |
Server preserves encoding |
31 |
Body with Unicode |
Include Unicode characters |
Server handles encoding correctly |
32 |
Body with Scripts |
Include scripts in body |
Server accepts but may flag as suspicious |
33 |
Body with Links |
Include hyperlinks |
Server accepts message |
34 |
Body with Large Content |
Send large body content |
Server accepts if within size limits |
35 |
Body with Empty Lines |
Include empty lines in body |
Server preserves formatting |
36 |
Body with Line Breaks |
Use line breaks correctly |
Server preserves formatting |
37 |
Header Injection Attempt |
Try injecting headers via body |
Server detects and blocks |
38 |
Body Injection Attempt |
Try injecting body via headers |
Server detects and blocks |
39 |
Header and Body Logging |
Log headers and body separately |
Logs show correct separation |
40 |
Header and Body Parsing |
Parse headers and body correctly |
Server processes each part accurately |
41 |
Header and Body Validation |
Validate structure before sending |
Server accepts valid format |
42 |
Header and Body Encoding |
Encode headers and body properly |
Server decodes and delivers correctly |
43 |
Header and Body Compression |
Compress message content |
Server decompresses correctly |
44 |
Header and Body Encryption |
Encrypt headers and/or body |
Server decrypts securely |
45 |
Header and Body Signature |
Sign headers and body |
Server verifies signature |
46 |
Header and Body Forwarding |
Forward message with intact structure |
Structure preserved |
47 |
Header and Body Filtering |
Apply filters based on headers/body |
Filters work correctly |
48 |
Header and Body Archiving |
Archive message with structure |
Structure preserved in archive |
49 |
Header and Body Rendering |
Render message in client |
Headers and body displayed correctly |
50 |
Header and Body Compliance |
Check RFC compliance |
Message meets standards |
Reliable Delivery - Testcases
S.No |
Test Case |
Description |
Expected Result |
|---|---|---|---|
1 |
Successful Email Delivery |
Send email to valid recipient |
Server responds with 250 OK |
2 |
Retry on Temporary Failure |
Simulate temporary DNS failure |
Server queues and retries |
3 |
Retry Interval Configuration |
Check retry timing |
Server retries after configured interval |
4 |
Max Retry Limit |
Exceed retry attempts |
Server bounces message |
5 |
Delivery Confirmation |
Request delivery status notification |
DSN received |
6 |
Message Queueing |
Queue message when recipient server is down |
Message stored for retry |
7 |
Message Persistence |
Server restart during queue |
Message remains queued |
8 |
Message Expiry |
Message exceeds queue lifetime |
Server returns failure |
9 |
Bounce Message Generation |
Delivery fails permanently |
Bounce message sent to sender |
10 |
Valid Recipient Address |
Send to valid address |
Message delivered |
11 |
Invalid Recipient Address |
Send to invalid address |
Server returns 550 error |
12 |
Domain Resolution |
Resolve recipient domain |
DNS lookup succeeds |
13 |
MX Record Lookup |
Lookup MX record for domain |
MX record found |
14 |
No MX Record |
Domain without MX record |
Server returns error |
15 |
Server Timeout Handling |
Recipient server times out |
Message queued for retry |
16 |
Network Failure Recovery |
Simulate network failure |
Server retries delivery |
17 |
Duplicate Message Prevention |
Prevent duplicate delivery |
Server checks message ID |
18 |
Message ID Tracking |
Track message ID across retries |
ID remains consistent |
19 |
Delivery Logging |
Log delivery attempts |
Logs show complete history |
20 |
Delivery Audit Trail |
Audit delivery path |
Trail is complete and accurate |
21 |
Delivery to Multiple Recipients |
Send to multiple valid addresses |
All messages delivered |
22 |
Partial Delivery Handling |
Some recipients invalid |
Valid ones receive message |
23 |
Delivery to Alias |
Send to email alias |
Message forwarded correctly |
24 |
Delivery to Mailing List |
Send to list address |
All members receive message |
25 |
Delivery to Forwarding Address |
Send to forwarding address |
Message redirected correctly |
26 |
Delivery with Attachment |
Send email with attachment |
Attachment received intact |
27 |
Delivery with Large Body |
Send large message body |
Delivered if within limits |
28 |
Delivery with HTML Content |
Send HTML email |
Rendered correctly by client |
29 |
Delivery with Special Characters |
Include special characters |
Delivered without corruption |
30 |
Delivery with Unicode |
Include Unicode characters |
Delivered correctly |
31 |
Delivery with MIME Format |
Use MIME formatting |
Server processes correctly |
32 |
Delivery with Priority Header |
Set priority in header |
Server respects priority |
33 |
Delivery with Read Receipt |
Request read receipt |
Receipt received if supported |
34 |
Delivery with DSN Options |
Use NOTIFY and RET parameters |
Server honors settings |
35 |
Delivery with Encryption |
Send over STARTTLS |
Message delivered securely |
36 |
Delivery with Authentication |
Authenticate before sending |
Server accepts message |
37 |
Delivery with SPF/DKIM/DMARC |
Use proper authentication records |
Message passes checks |
38 |
Delivery to External Domain |
Send to different domain |
Message routed correctly |
39 |
Delivery to Internal Domain |
Send within same domain |
Message delivered quickly |
40 |
Delivery with Retry Backoff |
Use exponential backoff |
Retry intervals increase |
41 |
Delivery with Queue Monitoring |
Monitor message queue |
Message status visible |
42 |
Delivery with Alerting |
Alert on delivery failure |
Notification sent to admin |
43 |
Delivery with Logging Enabled |
Enable detailed logs |
Logs show full delivery path |
44 |
Delivery with Logging Disabled |
Disable logging |
No logs generated |
45 |
Delivery with Failover Server |
Use backup SMTP server |
Message delivered via failover |
46 |
Delivery with Load Balancer |
Route through load balancer |
Message delivered efficiently |
47 |
Delivery with Rate Limiting |
Exceed rate limit |
Server throttles delivery |
48 |
Delivery with Quota Enforcement |
Exceed mailbox quota |
Server returns 552 error |
49 |
Delivery with Session Timeout |
Session expires during send |
Server terminates connection |
50 |
Delivery with Session Resume |
Resume interrupted session |
Message delivery continues |
Reference links