5.1.3 bounce: bad destination address syntax
5.1.3Bad destination mailbox address syntaxPermanent: retrying will not help
501 5.1.3 Bad recipient address syntax
What it means
RFC 3463 calls X.1.3 "Bad destination mailbox address syntax". The receiving server could not even parse the recipient as an email address, so it never got as far as checking whether a mailbox exists. The text "Bad recipient address syntax" above is exactly what Postfix returns for an unparseable RCPT TO.
It normally arrives with basic code 501 (syntax error in parameters) or 553, during the RCPT TO step. Typical triggers are spaces, a missing domain, two @ signs, unbalanced angle brackets, a trailing dot, or non-ASCII characters sent to a server that does not support SMTPUTF8 (RFC 6531). Compare it with 5.1.7, which is the same problem in the sender address, and with 5.1.1, where the syntax is fine but the mailbox is unknown.
A temporary 4.1.3 is rare, since bad syntax will not fix itself. When you do see 5.1.3 it is a permanent error and the message comes straight back.
Likely causes
- A space, comma, or semicolon inside the address, often from copying a list of addresses into one field.
- A missing or doubled @, or a domain ending in a dot such as user@example.com.
- An internationalised address (for example with accented letters) sent to a server that does not advertise SMTPUTF8.
- An application or script that builds RCPT TO lines without angle brackets or with a display name left inside.
How to fix it
- Copy the address from the bounce into a plain text editor and look for stray spaces, quotes, or invisible characters picked up from a web page or spreadsheet.
- Make sure each recipient sits in its own field or is separated with the delimiter your mail client expects.
- If the address contains non-ASCII characters, ask the recipient for an ASCII alias, or confirm that both your server and theirs support SMTPUTF8.
- If an application sends the mail, log the exact RCPT TO it issues and correct the code so it sends `RCPT TO:<user@example.com>` with only the address between the brackets.
Checks that apply
- SMTP Diagnostics Shows whether the recipient server advertises SMTPUTF8, which decides whether an internationalised address can be accepted.
- Email Header Analyzer Lets you inspect the To and envelope fields of the original message for hidden characters or malformed brackets.
FAQ
- Can a valid-looking address still get 5.1.3?
- Yes. Invisible characters such as a non-breaking space or a right-to-left mark make the address look normal but fail parsing on the server.
- Is 5.1.3 caused by the recipient’s server?
- Almost never. It reflects what your client or application sent, so the fix sits on the sending side.