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.
Should you remove the address?
Remove the address from your list
Fix it or remove it. As written the address can never be delivered, but a formatting error is sometimes recoverable when the intended address is clear.
Hard bounce. Do not retry. It will fail every time.
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.
Who sends it
Gmail documents it, and most servers send something similar. It usually points to a data problem in the list, such as a column import that merged two fields.
Gmail
Reply text The recipient address <address> is not a valid RFC 5321 address.
Gmail inserts the address it rejected into the reply.
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.
- Is 553 5.1.3 a hard bounce or a soft bounce?
- 553 5.1.3 is a hard bounce. The receiving side has said the address can't receive mail, so a retry won't deliver it. Do not retry. It will fail every time.
- Should I remove the address after a 553 5.1.3 bounce?
- Fix it or remove it. As written the address can never be delivered, but a formatting error is sometimes recoverable when the intended address is clear.
- Will verifying my list prevent 553 5.1.3?
- Yes. A syntax check catches these before anything is sent, and it is the first step of every verification run.
Clean the list before the next send.
A bounce report only tells you about the addresses you already mailed. The rest of the list has the same kinds of dead mailboxes in it, and each one becomes a hard bounce recorded against your domain the moment you send.
- A real SMTP mailbox check on every row, before anything is sent
- Dead and mistyped addresses split out, so they never reach your sending tool
- Catch-all, disposable and role addresses flagged alongside
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Other 553 replies
Sources
- Source: Google Workspace Admin Help: Gmail SMTP errors and codes
- Source: RFC 3463: Enhanced Mail System Status Codes
Provider wording is quoted only where the provider publishes it.
Last reviewed 2026-10-22