What tools can you use for SMTP debugging?

When delivery fails at the SMTP level, these tools help diagnose exactly what's happening.

telnet: Basic SMTP connection testing

telnet mail.example.com 25

Manually type SMTP commands to see server responses

No encryption; works for understanding protocol flow

openssl s_client: TLS-encrypted SMTP testing

openssl s_client -connect mail.example.com:465

openssl s_client -starttls smtp -connect mail.example.com:587

See certificate details and encrypted SMTP conversation

swaks (Swiss Army Knife for SMTP): Powerful SMTP testing tool

swaks --to recipient@example.com --from sender@yourdomain.com --server mail.example.com

Supports authentication, TLS, custom headers

Shows full SMTP transcript

mxtoolbox.com: Online SMTP diagnostics

Test MX records, SMTP connectivity, blacklist status

No software installation required

Wireshark: Network packet capture

Deep inspection of SMTP traffic

Useful for complex debugging

mail-tester.com: Send test emails and get deliverability report

Checks SPF, DKIM, content, blacklists

Easy to use for quick diagnostics

Start with swaks or telnet for most SMTP debugging. They reveal exactly what the server says in response to your connection and messages.