Analyzing SMTP Traffic with Wireshark for Security Engineers

Wireshark remains one of the most effective tools for protocol-level visibility. When dealing with email infrastructure, SMTP captures can reveal both normal behavior and configuration weaknesses with remarkable clarity.

This article focuses on how security and infrastructure engineers can analyze SMTP traffic in Wireshark to better understand service behavior and spot common issues.

Why Analyze SMTP Traffic?

SMTP packet analysis is useful for:

• Troubleshooting mail delivery problems
• Inspecting server capabilities
• Verifying STARTTLS negotiation
• Confirming authentication flows
• Teaching protocol fundamentals

Start with the Right Capture Filter

If you want to limit what is captured, common capture filters include:

tcp port 25
tcp port 587
tcp port 465

For analysis after capture, display filters are more flexible.

Useful Wireshark Display Filters

smtp
tcp.port == 25
tcp.port == 587
ip.addr == x.x.x.x
smtp.req.command
smtp.rsp.code

These filters help isolate SMTP traffic quickly in noisy enterprise captures.

Typical SMTP Session Flow

In a normal unencrypted SMTP exchange, look for:

1. TCP handshake
2. Server banner
3. EHLO or HELO
4. Optional STARTTLS
5. MAIL FROM
6. RCPT TO
7. DATA
8. Message body
9. QUIT

This transaction model is useful for both troubleshooting and detection engineering.

Interpreting the EHLO Response

EHLO often reveals the most useful operational details early in the session:

250-mailserver.example.com
250-PIPELINING
250-SIZE 52428800
250-STARTTLS
250-AUTH LOGIN PLAIN
250 HELP

Important fields include:

• STARTTLS – indicates TLS upgrade support
• AUTH – shows supported authentication methods
• SIZE – indicates message size policy
• PIPELINING – shows support for multiple queued commands

Security Observations to Look For

During analysis, engineers should pay attention to:

• Cleartext SMTP sessions where encryption is expected
• Missing STARTTLS on submission paths
• Weak or legacy AUTH methods
• Verbose banners revealing software details
• Unexpected relay behavior

These observations often indicate policy drift or incomplete hardening.

STARTTLS Validation

If the client issues STARTTLS and the server accepts, the SMTP payload should no longer remain readable in plaintext after the TLS handshake.

This is an important validation point:

• If traffic remains readable after expected encryption, configuration may be wrong
• If STARTTLS is advertised but negotiation fails, clients may fall back or fail delivery

Reading Response Codes

SMTP response codes provide operational meaning:

• 220 – Service ready
• 250 – Requested action completed
• 354 – Start mail input
• 421 – Service unavailable
• 450/451/452 – Temporary failure
• 550/551/552/553 – Permanent rejection conditions

When troubleshooting, these codes matter more than guesswork.

Training Use Cases

SMTP captures are excellent for trainee exercises because they are:

• Readable
• Sequential
• Protocol-rich
• Useful for correlating TCP and application behavior

A good exercise is to ask trainees to identify:

• The server banner
• The EHLO response
• Whether STARTTLS is offered
• The sender and recipient sequence
• The point where message body transmission begins

Operational Value in Enterprise Networks

In live environments, SMTP analysis helps with:

• Mail flow diagnostics
• Intermittent submission failures
• Transport security validation
• Firewall policy verification
• Application integration troubleshooting

For engineering teams, packet-level visibility often closes the gap between assumption and evidence.

Conclusion

SMTP analysis in Wireshark is practical, direct, and still highly relevant. For infrastructure and security engineers, it provides a disciplined way to verify service behavior, hardening posture, and protocol correctness.