Permanent failure — do not retry
SMTP 555
MAIL FROM or RCPT TO parameters not recognised
SMTP 555 means the server did not recognise or does not support a parameter on MAIL FROM or RCPT TO. RFC 5321 defines it as "MAIL FROM/RCPT TO parameters not recognized or not implemented". It is a client or formatting fault, and often a malformed address in angle brackets.
How it appears
555 MAIL FROM/RCPT TO parameters not recognized or not implemented
555 5.5.4 Unsupported option
555 5.5.2 Syntax error.
What causes it
- The client added an extension parameter, such as SMTPUTF8 or SIZE, that the server did not advertise.
- The address in MAIL FROM or RCPT TO is malformed or contains extra text after the closing bracket.
- A library is building the envelope incorrectly.
How to fix it
In order — try the first one first.
- Look at the exact MAIL FROM or RCPT TO line in the session log.
- Only send extension parameters the server listed in its EHLO response.
- Clean the address: remove display names, quotes and trailing characters from the envelope address.
Common questions
- What does SMTP 555 mean?
- SMTP 555 means the server did not recognise or does not support a parameter on MAIL FROM or RCPT TO. RFC 5321 defines it as "MAIL FROM/RCPT TO parameters not recognized or not implemented". It is a client or formatting fault, and often a malformed address in angle brackets.
- Should I retry a 555 error?
- No. 555 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 555?
- Look at the exact MAIL FROM or RCPT TO line in the session log.
Last reviewed 2026-09-11