Basic SMTP reply code · RFC 5321

502 Command not implemented

The server does not implement this command.

Class: 5yz · Permanent failure
5yz: a permanent failure. The same request should not be repeated; something must change first, such as the address (RFC 5321 §4.2.1).
Associated enhanced codes
The enhanced codes that usually come with this code, per the registry and the providers:

5.5.05.5.25.5.45.7.15.5.1
Source
RFC 5321

How 502 appears at providers

The exact text from the provider's documentation, as you see it in the bounce, and what it means.

502 5.5.1Unimplemented command.

The SMTP client sent a command that is unknown, not implemented or out of order (RFC 5321). The problem is in the sending software.
502 5.5.1Unrecognized command.

The SMTP client sent a command that is unknown, not implemented or out of order (RFC 5321). The problem is in the sending software.
502 5.5.1Too many unrecognized commands, goodbye.

The SMTP client sent a command that is unknown, not implemented or out of order (RFC 5321). The problem is in the sending software.

Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026