550 5.7.750: sending from unregistered domains

What it means

Microsoft 365 lets a tenant send only a limited amount of mail from domains that are not among its accepted domains. When a large volume comes from such unregistered domains, Exchange Online blocks the tenant with 5.7.750. The usual cause is a server or application relaying mail for domains nobody added to the tenant. Less often, connectors are configured incorrectly, and attackers sometimes create inbound connectors in a tenant to send spam. After fixing the cause, an admin asks Microsoft Support to remove the block.

Common causes

• Mail is sent through the tenant from domains that are not accepted domains.
• One or more connectors are configured incorrectly.
• An attacker created an inbound connector in the tenant to relay spam.

How to fix it

1. 01Add and validate in Microsoft 365 every domain you send from, which Microsoft calls the most common solution.
2. 02Use a certificate-based outbound connector whose certificate domain is an accepted and validated domain.
3. 03Look for unfamiliar connectors and compromised accounts, and remove them.
4. 04Contact Microsoft Support to lift the block once the cause is fixed.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• Fix NDR error codes 550 5.7.703, 550 5.7.705, 550 5.7.708, and 550 5.7.750learn.microsoft.com
• Email nondelivery reports and SMTP errors in Exchange Onlinelearn.microsoft.com

Further reading

• The standard meaning of 550: mailbox unavailable