550 5.1.1 User unknown
The recipient address doesn't exist
Seen in All mail servers
The receiving server has no mailbox for that address.
Common causes
- A typo in the address.
- The mailbox was deleted or the person left.
- MX points at a server that doesn't know the domain's users.
How to fix it
- Check the spelling with the recipient.
- Remove the address from mailing lists.
- If it's your domain, check MX records and mailbox provisioning.
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Related errors
- 550 5.1.10 RESOLVER.ADR.RecipientNotFound · Microsoft 365 could not find the recipient
- 550 5.4.4 SMTPSEND.DNS.NonExistentDomain; nonexistent domain · The recipient's domain does not exist in DNS
- 554 5.7.1 <user@example.com>: Relay access denied · Postfix refused to relay to that recipient
- 450 4.1.8 <user@example.com>: Sender address rejected: Domain not found · The sender's domain does not resolve in DNS
- 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
- 554 5.7.1 Service unavailable; Client host blocked using a blocklist · The sending IP is on a blocklist
- 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 5.4.1 Recipient address rejected: Access denied · Microsoft 365 rejected an unknown recipient at the edge