550 5.7.1 Relaying denied / Message rejected
The receiving server refused the message on policy grounds
Seen in Exchange, Postfix, most mail servers
A generic policy rejection: the server won't relay for you, or it rejected the message for security or reputation reasons.
Common causes
- Sending through a server without authenticating (relay denied).
- The sending IP or domain is on a blocklist.
- SPF/DMARC failure or recipient-side rules.
How to fix it
- Authenticate to your mail server on port 587 before sending.
- Check the sending IP against blocklists.
- Check SPF, DKIM and DMARC for the From domain.
Got a different message? Paste it into the error explainer.
Related errors
- 550 5.7.26 Unauthenticated email is not accepted · Gmail rejected mail that failed DMARC
- 421 4.7.0 Try again later · Temporarily deferred by the receiving server
- 550 5.7.515 Access denied, sending domain does not meet the required authentication level · Outlook.com rejected unauthenticated bulk mail
- 550 5.7.23 SPF validation failed · The message failed the SPF check
- 550 5.7.509 Access denied, sending domain does not pass DMARC verification and has a DMARC policy of reject · Microsoft 365 rejected mail that failed DMARC
- 421 4.7.28 Our system has detected an unusual rate of unsolicited mail originating from your IP address · Gmail is rate limiting mail from your IP
- 550 5.7.1 Our system has detected that this message is likely unsolicited mail · Gmail blocked the message as spam
- 421 4.7.26 This mail has been rate limited because it is unauthenticated · Gmail deferred mail that passed neither SPF nor DKIM
- 421 4.7.27 Your email has been rate limited because SPF authentication didn't pass for this message · Gmail deferred or blocked mail that failed SPF
- 421 4.7.0 [TSS04] Messages from x.x.x.x temporarily deferred due to unexpected volume or user complaints · Yahoo is temporarily deferring your mail
- 550 5.7.25 The IP address sending this message does not have a PTR record · The sending IP has no reverse DNS
- 550 5.1.1 User unknown · The recipient address doesn't exist