550 5.7.1 Microsoft bounce: unable to relay
What 550 5.7.1 means
The receiving system will not relay mail for a domain it does not accept, so it refuses the anonymous message.
Message text
Unable to relay
Common causes of 550 5.7.1
- MX of the recipient domain points to a server that does not accept that domain
- Sending system tried to relay through a server that does not allow it
How to fix 550 5.7.1
- Run MX lookup on the recipient domain.
- If you send through a relay or provider, configure SMTP authentication for it.
Check your domain
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Have the full bounce message? Paste it into the bounce code lookup and it will match the code and the provider for you. Open the bounce code lookup.
How to read 550 5.7.1
The first part, 550, is the SMTP reply code. Requested action not taken, mailbox unavailable: the mailbox does not exist, is not allowed, or the message was refused for policy reasons. Replies that start with 4 are temporary and replies that start with 5 are permanent (RFC 5321 §4.2.1).
The second part, 5.7.1, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 5 is a Permanent failure (5xx): the command was rejected and the sender should not repeat it unchanged (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 7 is security or policy: authentication, authorization or policy refused the message. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.
Do not keep resending the same message: a permanent failure repeats until the cause is fixed. Correct the cause, then send a new message.
Official source: learn.microsoft.com. Last verified 2026-10-20.
Related guides
- How to create an SPF record (syntax and examples)
- How DKIM works and how to publish a DKIM record
- How to set up a DMARC record, from p=none to p=reject
Related codes
- 5.7.1 SMTP: delivery not authorized
- 550 5.7.1 Gmail: likely unsolicited
- 550 5.7.1 Microsoft: delivery not authorized
- 530 5.7.1 Microsoft: not authenticated
- 5.7.0 SMTP: security status
- 5.7.23 SMTP: SPF validation failed
FAQ
What does 550 5.7.1 mean?
The receiving system will not relay mail for a domain it does not accept, so it refuses the anonymous message. This is a permanent failure.
How do I fix 550 5.7.1?
Run MX lookup on the recipient domain. If you send through a relay or provider, configure SMTP authentication for it.
Is 550 5.7.1 permanent, and should I retry?
550 5.7.1 is a permanent failure, so retrying the same message will fail again. Fix the cause first, then send a new message.