451 4.5.0: SMTP protocol violation
Your server sent Gmail something that breaks the rules of the SMTP standard, RFC 5321, so Gmail stopped the transaction with a temporary error.
- Gmail
- Provider
- Temporary (4xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 451 4.5.0
- Provider
- Gmail and Google Workspace
- Temporary or permanent
- TemporaryThe sending server keeps the message and tries again later.
- Who has to act
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
451 4.5.0 SMTP protocol violation.
What it means
Gmail found the SMTP conversation itself malformed: a command or data arrived in a form, or at a moment, that the standard does not allow. Google gives no more detail and refers senders to RFC 5321. This points at the sending software, not at content or reputation. Home-grown SMTP clients, line endings other than CRLF, bare line feeds in the message body, and devices such as printers or scanners with old mail code are the usual suspects. Because the code is temporary the message is retried, but each retry fails the same way until the client is fixed.
Common causes
- A custom or outdated SMTP client that does not follow RFC 5321.
- Line endings other than CRLF, or bare line feeds in the message.
- A device such as a printer or scanner with limited mail software.
How to fix it
- 01Find which client or device sent the message, using your server logs and the time of the bounce.
- 02Update the sending software, or switch it to a maintained mail library.
- 03Make sure every line, including those in the message body, ends with CRLF.
- 04Route devices that cannot be fixed through a mail server that cleans up their output before it reaches Gmail.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Gmail SMTP errors and codes (Google Workspace Admin Help)knowledge.workspace.google.com
Related codes
- 555 5.5.2Syntax error in a command.Gmail could not parse a command or its parameters from your client, usually MAIL FROM or RCPT TO, and refused it.
- 451Local error in processing.The receiving server hit a problem on its own side while handling the message and stopped. The reply is temporary, so the sending server will try again.
- 5.5.0SMTP protocol problem, unspecified.Something went wrong in the SMTP conversation between the servers that the receiving side did not describe with a more specific protocol code.
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