Permanent failure — do not retry
SMTP 553
mailbox name not allowed
SMTP 553 means the server will not accept the mailbox name. RFC 5321 defines it as "requested action not taken: mailbox name not allowed", for example because the syntax is wrong. It is used for malformed recipient addresses, and by some servers for sender addresses you are not allowed to use.
How it appears
553 Requested action not taken: mailbox name not allowed
553 5.1.3 The recipient address is not a valid RFC 5321 address.
553 5.7.1 Sender address rejected: not owned by user
What causes it
- The recipient address has invalid syntax.
- The sender address in MAIL FROM is not one the authenticated account may send as.
- The recipient domain does not exist or is not one the server accepts.
- The server refuses to relay to the destination.
How to fix it
In order — try the first one first.
- Check the address in the failing command for typos, spaces and invalid characters.
- If the rejection is on MAIL FROM, send from an address your account owns or is permitted to send as.
- Remove recipient addresses that are malformed or on domains that do not exist.
Common questions
- What does SMTP 553 mean?
- SMTP 553 means the server will not accept the mailbox name. RFC 5321 defines it as "requested action not taken: mailbox name not allowed", for example because the syntax is wrong. It is used for malformed recipient addresses, and by some servers for sender addresses you are not allowed to use.
- Should I retry a 553 error?
- No. 553 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 553?
- Check the address in the failing command for typos, spaces and invalid characters.
Specific 553 replies
Last reviewed 2026-09-11