421 4.7.0 Try again later
Temporarily deferred by the receiving server
Seen in Gmail, Outlook, most mail servers
The receiver temporarily refused the connection or message; your server should retry.
Common causes
- Rate limiting because of volume or a sudden spike.
- Low sender reputation or missing authentication.
- TLS required by the receiver but not negotiated.
How to fix it
- Let your server retry with backoff; reduce sending rate.
- Set up SPF, DKIM, DMARC and reverse DNS.
- Enable STARTTLS for outbound mail.
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
- 550 5.7.1 Relaying denied / Message rejected · The receiving server refused the message on policy grounds
- 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