502: command not implemented
The server recognises the command but does not implement it, so it will not carry that command out in this session.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 502
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
502 Command not implemented
What it means
RFC 5321 separates two cases: 500 for a command the server does not recognise, and 502 for one it recognises but has not implemented. Servers also answer 502 to an extension they never offered, and some answer it to VRFY or EXPN when they have switched those off, although RFC 5321 asks for 252 in that case. It refuses that one command and usually leaves the rest of the session working. RFC 5321 forbids servers to list in their EHLO reply a capability they would refuse with 502, so a client that reads the EHLO list should not trigger it.
Common causes
- A client used VRFY, EXPN or another command that the server has switched off or does not implement.
- Software used an extension without checking the server's EHLO reply.
How to fix it
- 01Find the command that got the 502 in the session log.
- 02Use only the commands and extensions the server lists in its EHLO reply.
- 03Update the sending software if it assumes features the server does not have.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- RFC 5321, section 4.2.3: Reply Codes in Numeric Orderrfc-editor.org
- RFC 5321, section 4.2.4: Reply Code 502rfc-editor.org
Related codes
- 500Command not recognised.The server did not recognise the command it received, or the command line was too long. This is a permanent error in the SMTP conversation.
- 504Command parameter not implemented.The server implements the command, but not the particular parameter or option the client asked for with it.
- 5.5.1Invalid SMTP command.The sending software issued an SMTP command that the server does not support, or sent it at the wrong point in the conversation.
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