500: command 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.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 500
- 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
500 Syntax error, command unrecognized
What it means
RFC 5321 uses 500 for a command the server does not recognise, including a command line that is too long. It points at the client side: a typo in a manual SMTP session, software sending a command the server does not know, or something on the path changing the traffic. RFC 5321 expects servers to answer unknown commands with 500 and keep the session open, so a single 500 does not stop delivery by itself unless the client gives up. RFC 4954 pairs 500 with 5.5.6 for an authentication line that is too long.
Common causes
- Sending software that issues a command the server does not support.
- A command line longer than the server accepts.
- A firewall or proxy that alters SMTP traffic on the way.
How to fix it
- 01Look in the SMTP session log for the command that received the 500.
- 02Update or reconfigure the sending software.
- 03If a firewall inspects SMTP between the servers, turn that inspection off and test again.
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
- RFC 5321, section 3.8: Terminating Sessions and Connectionsrfc-editor.org
Related codes
- 501Syntax error in arguments.The command was recognised, but what came with it was malformed, often an address or a host name the server cannot accept.
- 502Command not implemented.The server recognises the command but does not implement it, so it will not carry that command out in this session.
- 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.
- 5.5.2SMTP syntax error.A command from the sending software could not be understood, because its syntax was wrong or the command was not recognised at all.
- 500 5.5.6Authentication data too long.During login, the client sent an authentication response longer than the server can accept for the chosen method, so the AUTH command failed.
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