553 5.1.3 Bad recipient address syntax
The recipient address is not a valid email address as far as the server can tell: a missing @, two @ signs, spaces, quotes in the wrong place, a trailing dot, or characters outside what the server permits. The server never looked the mailbox up; it could not parse the address well enough to try.
What the server replies
501 5.1.3 Bad recipient address syntax
- Sent by
- Any mail server
- Type
- PermanentRetry fails
- About
- Recipient problems
- Enhanced code
- 5.1.3
Who sends it
Postfix returns 501 5.1.3 at RCPT TO; Exchange returns 553 5.1.3 "Invalid address" and 501 5.1.3 for addresses with unsupported characters; most servers reject before any lookup.
Why it happens
- A form that lets users enter "name <user@example.com>" or two addresses in one field.
- Copy-pasted addresses with a leading or trailing space, or invisible characters.
- Non-ASCII characters in the local part when the server does not support SMTPUTF8.
- Angle brackets, commas or semicolons left over from an address book export.
How to fix it
Validate before sending
Trim whitespace, strip display names, and check the address against a sane pattern (one @, a domain with a dot, no spaces) before it reaches the SMTP layer.
Handle international addresses explicitly
If you need to send to addresses with non-ASCII local parts, check that the server advertises SMTPUTF8 in its EHLO reply and that your library sends the option.
Look at the raw transaction
Log the exact RCPT TO line the client sent. The malformed part is usually obvious once you see it.
Check it yourself
These free tools show the records and connections behind this code for your own domain or server.
Related codes
- 501 5.5.4 Invalid HELO/EHLO argument / syntax error in parametersThe server did not like the arguments your client sent with a command, most often the hostname in HELO or EHLO.
- 550 5.1.1 User unknown / recipient address rejectedThe most common bounce there is: the mailbox does not exist.
- 550 5.1.2 Host unknown / bad destination system addressThe domain part of the recipient address is the problem: it does not resolve, so no server can be found to deliver to.
Sources
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