Standard SMTP codes · permanent error
555: parameters not recognised
The server does not recognise, or does not implement, a parameter given with MAIL FROM or RCPT TO, so that command failed.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 555
- 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
555 MAIL FROM/RCPT TO parameters not recognized or not implemented
What it means
RFC 5321 says a server returns 555 when it does not recognise or cannot implement one or more parameters on a MAIL FROM or RCPT TO command. Those parameters are extension options such as SIZE, BODY or NOTIFY, which a client should only send when the server listed the matching extension in its EHLO reply. 555 is permanent for that command; 455 is its temporary counterpart. It usually means the sending software added an option the server never offered.
Common causes
- Sending software adds an extension parameter without checking the EHLO reply.
- A relay on the path supports fewer extensions than the software expected.
How to fix it
- 01Read the reply text for the parameter concerned.
- 02Make sure the software only sends parameters for extensions the server advertises.
- 03Resend without that option.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 455Parameters cannot be handled now.The server cannot handle one of the parameters given with MAIL FROM or RCPT TO at the moment, and asks the sender to try again later.
- 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.
- 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.
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