504: command parameter not implemented
The server implements the command, but not the particular parameter or option the client asked for with it.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 504
- 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
504 Command parameter not implemented
What it means
RFC 5321 describes 504 as a refinement of 502: the command exists, but a parameter requested with it is not implemented. The common case is AUTH with a mechanism the server does not support; RFC 4954 prescribes 504, with enhanced code 5.5.4, for an authentication mechanism that is unsupported or needs encryption first. The registry also pairs 504 with 5.7.4 for unsupported security features. It is permanent for that command, and choosing an option the server does offer fixes it.
Common causes
- AUTH with a mechanism the server does not offer.
- A command option that this server does not implement.
How to fix it
- 01Read the server's EHLO reply for the AUTH mechanisms and extensions it supports.
- 02Configure the client to use one of those.
- 03Update the client if it picks mechanisms on its own and chooses one the server lacks.
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 4.2.1: Reply Code Severities and Theoryrfc-editor.org
- RFC 4954, section 4: The AUTH Commandrfc-editor.org
Related codes
- 502Command not implemented.The server recognises the command but does not implement it, so it will not carry that command out in this session.
- 5.5.4Invalid command arguments.A valid SMTP command was sent with an argument the server does not accept, because the value was out of range or named a feature it does not know.
- 5.7.4Security feature not supported.The message or session asked for a security feature, such as a form of secure authentication, that the delivery protocol here cannot support.
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