Permanent failure — do not retry
SMTP 502
command not implemented
SMTP 502 means the server recognises the command but does not support it. RFC 5321 defines it as "command not implemented". It usually appears when a client uses an optional command, such as VRFY, EXPN or AUTH, that the server has switched off. It is not a verdict on any address.
How it appears
502 Command not implemented
502 5.5.1 VRFY command is disabled
502 5.5.2 Error: command not recognized
What causes it
- The client sent VRFY or EXPN and the server disables them, as most do.
- The client tried AUTH on a server or port that does not offer authentication.
- The client sent an extended command without first checking the EHLO response for support.
How to fix it
In order — try the first one first.
- Check which extensions the server lists in its EHLO response and only use those.
- If AUTH is rejected, connect to the submission port your provider documents, usually 587 with STARTTLS.
- Do not rely on VRFY to check addresses. Most servers disable it, which is why verifiers use RCPT TO instead.
Common questions
- What does SMTP 502 mean?
- SMTP 502 means the server recognises the command but does not support it. RFC 5321 defines it as "command not implemented". It usually appears when a client uses an optional command, such as VRFY, EXPN or AUTH, that the server has switched off. It is not a verdict on any address.
- Should I retry a 502 error?
- No. 502 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 502?
- Check which extensions the server lists in its EHLO response and only use those.
Last reviewed 2026-09-11