554 5.7.1 Service unavailable; Client host blocked using a blocklist
The sending IP is on a blocklist
Seen in Postfix, Exchange, Spamhaus-protected servers
The receiving server checked your sending IP against a DNS blocklist and found it listed.
Common causes
- A compromised account or server sending spam.
- A shared or recycled IP with a poor history.
- A dynamic/residential IP sending mail directly.
How to fix it
- Find and stop the source of spam, then request delisting from the list named in the bounce.
- Send through a reputable relay or your provider's smart host.
- Make sure reverse DNS and SPF are correct.
Got a different message? Paste it into the error explainer.
Related errors
- 550 5.7.606 Access denied, banned sending IP · Microsoft banned the sending IP address
- 421 4.7.0 [TSS09] All messages from x.x.x.x will be permanently deferred; Retrying will NOT succeed · Yahoo has blocked your sending IP
- 550 5.7.26 Unauthenticated email is not accepted · Gmail rejected mail that failed DMARC
- 550 5.7.25 The IP address sending this message does not have a PTR record · The sending IP has no reverse DNS
- 550 5.7.1 Relaying denied / Message rejected · The receiving server refused the message on policy grounds
- 550 5.1.1 User unknown · The recipient address doesn't exist
- 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 5.1.10 RESOLVER.ADR.RecipientNotFound · Microsoft 365 could not find the recipient
- 550 5.4.1 Recipient address rejected: Access denied · Microsoft 365 rejected an unknown recipient at the edge
- 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