How to Debug SMTP Connection Issues Like a Pro
When your emails aren't sending, the SMTP connection log is your best friend. Understanding how to read and interpret SMTP logs turns guesswork into diagnosis. This guide teaches you to debug SMTP issues like a professional email administrator.
Understanding SMTP Response Codes
- 220: Server ready — connection established successfully
- 221: Server closing connection
- 235: Authentication successful
- 250: Requested action completed (OK)
- 334: Server accepts authentication (waiting for credentials)
- 421: Service not available — server temporarily overloaded
- 451: Local error — try again later
- 454: TLS not available
- 530: Authentication required
- 535: Authentication credentials invalid
- 550: Requested action not taken — mailbox unavailable or access denied
- 554: Transaction failed — message rejected
Using PayPaell SMTP Checker for Debugging
PayPaell's SMTP Checker provides a detailed connection log showing every step of the SMTP handshake. When something fails, the log shows exactly which response code was returned and where the connection broke — making diagnosis fast and accurate.
Common Debugging Scenarios
Connection Times Out
The server isn't responding. Check hostname, port, firewall settings, and network connectivity.
Auth Fails at Step 2
Credentials are wrong. For Gmail, you may need an App Password. Check your email provider's documentation.
Connected but Emails Don't Arrive
Connection works but delivery fails. Usually an authentication, SPF, or DKIM issue. Check your DNS records.
Conclusion
SMTP debugging is a systematic process. Read the response codes, identify the failure point, and fix accordingly. PayPaell's SMTP Checker provides the diagnostic detail you need. Combine with PayPaell's Email Validator for complete email health.