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Standard SMTP codes · permanent error

5.1.3: recipient address is malformed

The recipient address breaks the rules for how an email address must be written, so the server refused it before checking whether it exists.

Standard
Provider
Permanent (5xx)
Class
Sources checked
By , email deliverability consultant
Author

At a glance

Code
5.1.3
Temporary or permanent
PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
Who has to act
The sender. The fix is on the sending side: your domain, your server or your list.
What the server says
5.1.3 Bad destination mailbox address syntax

What it means

5.1.3 concerns the way the recipient address is written. The server could not parse it: a space, a missing @, two dots in a row, stray brackets, or characters the server does not allow. RFC 3463 reserves it for permanent failures, and it often travels with reply code 501 (syntax error in arguments) or 553 (mailbox name not allowed). Nothing about the address changes on a retry, so the message is returned. When every recipient of a message fails this way, the likely cause is the sending software building the RCPT TO command wrongly.

Common causes

  • A typo that leaves the address without an @, with spaces, or with two dots in a row.
  • Characters the receiving server does not accept, including non-ASCII letters on a server without SMTPUTF8.
  • A sending tool or script that quotes or wraps the address wrongly in the RCPT TO command.

How to fix it

  1. 01Look at the exact address quoted in the bounce and correct it.
  2. 02Check address format where addresses enter your system (forms, imports) before mail is sent.
  3. 03If every recipient fails, check how your application or SMTP library writes RCPT TO.
  4. 04For an address with non-ASCII characters, ask the recipient for an ASCII address if the server lacks SMTPUTF8.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

  1. 501Syntax error in arguments.The command was recognised, but what came with it was malformed, often an address or a host name the server cannot accept.StandardPermanent
  2. 553Mailbox name not allowed.The server refused the address because the mailbox name is not allowed there, most often because it is written incorrectly.StandardPermanent
  3. 5.1.7Sender address is malformed.The sender address in the envelope (MAIL FROM) is written in a way the receiving server cannot accept, so it refused the message.StandardPermanent
  4. 5.6.7Non-ASCII address not allowed.An address in the envelope contains non-ASCII characters (an internationalised address), and the receiving server will not accept that for this sender or recipient.StandardPermanent

5.1.3 at other providers

  1. 553 5.1.3Recipient address is not valid.Gmail rejected the recipient address because it is not written in a form the SMTP standard allows, so nothing can be sent to it.GmailPermanent

All 105 standard codesEvery code in the wikiPaste your own bounce

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