Permanent failure — do not retry
SMTP 553 5.1.3
invalid address syntax
SMTP 553 5.1.3 means the recipient address is not a valid email address. RFC 3463 defines X.1.3 as "bad destination mailbox address syntax", and Gmail phrases it as "The recipient address is not a valid RFC 5321 address." The server never looked up the mailbox, because the address itself is malformed.
How it appears
553 5.1.3 The recipient address <user@@example.com> is not a valid RFC 5321 address.
501 5.1.3 Bad recipient address syntax
What causes it
- A space, comma or other invalid character inside the address.
- Two @ signs, a missing @, or a missing domain part.
- A display name or angle brackets copied into the address field.
- Invisible characters pasted in from a spreadsheet or document.
How to fix it
In order — try the first one first.
- Look at the exact address in the bounce and correct the syntax.
- Clean the whole list for the same problem: trim whitespace and strip invisible characters before importing.
- Add syntax validation to your signup form so malformed addresses never get in.
Common questions
- What does SMTP 553 5.1.3 mean?
- SMTP 553 5.1.3 means the recipient address is not a valid email address. RFC 3463 defines X.1.3 as "bad destination mailbox address syntax", and Gmail phrases it as "The recipient address is not a valid RFC 5321 address." The server never looked up the mailbox, because the address itself is malformed.
- Should I retry a 553 5.1.3 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 5.1.3?
- Look at the exact address in the bounce and correct the syntax.
Other 553 replies
Last reviewed 2026-09-11