Permanent failure — do not retry
SMTP 501
syntax error in parameters or arguments
SMTP 501 means the command was recognised but its parameters were not valid. RFC 5321 defines it as "syntax error in parameters or arguments". The most common real cause is a malformed email address in MAIL FROM or RCPT TO, or a bad hostname in HELO.
How it appears
501 Syntax error in parameters or arguments
501 5.1.3 Bad recipient address syntax
501 5.5.4 Syntax: MAIL FROM:<address>
What causes it
- The recipient or sender address has invalid syntax, such as a space, a missing @ or two dots in a row.
- The address was not wrapped in angle brackets the way the server expects.
- The HELO or EHLO hostname is empty or not a valid domain name.
- Hidden characters, like a non-breaking space or a trailing comma, were copied into the address.
How to fix it
In order — try the first one first.
- Look at the exact address in the failing command. A 501 on RCPT TO almost always means the address is malformed.
- Clean the list: trim whitespace, remove invisible characters and reject anything that is not a valid address.
- Make sure your server sends a fully qualified hostname in HELO or EHLO.
Common questions
- What does SMTP 501 mean?
- SMTP 501 means the command was recognised but its parameters were not valid. RFC 5321 defines it as "syntax error in parameters or arguments". The most common real cause is a malformed email address in MAIL FROM or RCPT TO, or a bad hostname in HELO.
- Should I retry a 501 error?
- No. 501 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 501?
- Look at the exact address in the failing command. A 501 on RCPT TO almost always means the address is malformed.
Last reviewed 2026-09-11