5.7.1: unable to relay
The receiving system refused to pass the message on because it is not the final destination for the recipient domain.
- Microsoft
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.7.1
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
5.7.1 Unable to relay
What it means
This form of 5.7.1 comes from a receiving system asked to deliver anonymous mail for a domain it does not accept. Accepting it would make the system an open relay, so it refuses. Microsoft lists three common reasons: a spammer trying to relay through the system with your address forged as the sender, so the bounce lands with you; an MX record pointing to a system that does not accept that domain; and a sending system or client that should relay through it but lacks the permission.
Common causes
- A spammer tried to relay through the server using your address as the forged sender.
- The domain's MX record points to a system where the domain is not accepted.
- An application or device that relays through the server is not authorized to.
How to fix it
- 01If you did not send the original message, the report is probably backscatter and can be ignored.
- 02If you did, check the recipient domain's MX record and tell the recipient's admin.
- 03Domain admins: correct the MX record or accept the domain on the system it points to.
- 04For your own devices and applications, give them permission to relay, for example through a connector, or have them authenticate.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com
- Fix NDR error "550 5.7.1" in Exchange Onlinelearn.microsoft.com
Related codes
- 5.7.1IP listed on both a connector and the IP Allow list.An outside server's IP is set on both an inbound connector and the IP Allow list, which hides the real blocklist that refused it.
- 5.4.1Server does not accept mail for this domain.The server that answered does not accept mail for the recipient's domain, which usually points to a wrong MX record or server setting.
- 550 5.7.367Forwarded mail rejected by the next hop.A message Microsoft 365 forwarded or relayed was refused by the next server, often a third-party gateway, because it left from Microsoft's relay pool.
5.7.1 at other providers
- 550 5.7.1Blocked by a domain policy.A rule set by the recipient's or the sender's Google Workspace admin blocks this message, so Gmail rejected it under that policy.
- 550 5.7.1Relay IP does not match the sending domain.Google's SMTP relay knows your IP, but the mail's sending domain is not one that IP is registered to relay for in your Workspace account.
- 550 5.7.1Blocked as likely spam.Gmail's filters judged this message likely to be spam and rejected it outright, instead of delivering it to the spam folder.
- 550 5.7.1IPv6 sender without PTR or authentication.Gmail rejected mail that arrived over IPv6 because the sending address lacks a matching PTR record, or the message failed authentication.
- 550 5.7.1Blocked for low IP reputation.Gmail permanently rejected the message because the IP it came from has a very low reputation with Gmail.
- 550 5.7.1Blocked for low domain reputation.Gmail permanently rejected the message because the domain it comes from has a very low reputation with Gmail.
Further reading
All 111 Microsoft codesEvery code in the wikiPaste your own bounce