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.