Email provider · 4.x.x transient failure
Every SMTP 421 response on record from Google (gmail.com, googlemail.com), with the verbatim text, the enhanced status code, a severity rating and a plain-English fix.
The service is temporarily unavailable and the connection is being closed. The receiver is telling the sender to back off and reconnect shortly. It usually comes from rate limiting, a full queue, or a brief reputation check on an unfamiliar sender.
smtp;421 4.7.0 [x.xx.xx.xx] Our system has detected that this message is suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information. - gsmtp
The receiver's spam filter is deferring the message on content or reputation. Tighten the content and slow your ramp; persistent deferrals become rejections.
Provider documentation
smtp;421 4.7.0 [x.xx.xx.xx] Our system has detected that this message is suspicious due to the nature of the content and/or the links within. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information. - gsmtp
The receiver's spam filter is deferring the message on content or reputation. Tighten the content and slow your ramp; persistent deferrals become rejections.
Provider documentation
421 4.7.28 [x.xx.xx.xx] Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines. - gsmtp
The receiver's spam filter is deferring the message on content or reputation. Tighten the content and slow your ramp; persistent deferrals become rejections.
Provider documentation
4.7.32 This mail has been rate limited because there is no DMARC alignment
The message is being deferred over DMARC alignment. Confirm DKIM signs for your From domain, or that SPF is aligned via a custom Return-Path.
Provider documentation
Postmaster & documentation
The service is temporarily unavailable and the connection is being closed. The receiver is telling the sender to back off and reconnect shortly. It usually comes from rate limiting, a full queue, or a brief reputation check on an unfamiliar sender. At Google, it is a temporary deferral that is retried automatically, and the specific reason is in the enhanced status code and message text of each response below.
Match your bounce text to one of the responses below and follow its guidance. Recipient-side wording means fix the address. Authentication or reputation wording (SPF, DKIM, DMARC, blocklist) means fix your sending setup, and check Google's postmaster documentation.
