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
Seen in Yahoo Mail, AOL
Despite the 421 code, Yahoo has blocked the IP: every message from it will be refused until the reputation problem is fixed.
Common causes
- Sustained spam complaints or spam-trap hits from the IP.
- A compromised account or server sending spam.
- An IP with a poor history on a shared or recycled range.
How to fix it
- Stop the source of unwanted mail before asking for review.
- Clean the list and remove unengaged recipients.
- Request review through the Yahoo Sender Hub once fixed, or send through a reputable relay.
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Related errors
- 554 5.7.1 Service unavailable; Client host blocked using a blocklist · The sending IP is on a blocklist
- 550 5.7.606 Access denied, banned sending IP · Microsoft banned the sending IP address
- 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