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550 5.7.750: unregistered domains blocked in Microsoft 365

550 5.7.750Client blocked from sending from unregistered domainsPermanent: retrying will not helpFrom Microsoft 365 and Outlook.com

Example
550 5.7.750 Service unavailable. Client blocked from sending from unregistered domains.

What it means

Exchange Online expects a tenant to send from domains it has added and verified as accepted domains. When too much outbound mail uses a From or envelope domain that is not registered in the tenant, Microsoft blocks those messages with 5.7.750.

It typically hits organizations that relay through Microsoft 365 for several brands or for customers, or that kept an old domain in use after removing it from the tenant. It is different from a tenant threshold ban (5.7.705): the trigger here is the unprovisioned domain, not general spam.

The tenant admin fixes it by registering the domains or stopping that relay pattern.

Likely causes

  • A device or app relays mail through your connector using a domain you never added to Microsoft 365.
  • You host mail for customers or subsidiaries whose domains are not accepted domains in your tenant.
  • A domain was removed from the tenant but systems still send as it.
  • A compromised relay is sending as random domains.

How to fix it

  1. Run a message trace to see which sender domains are being rejected.
  2. Add each legitimate domain in the Microsoft 365 admin center and verify it with the TXT record Microsoft gives you, such as `MS=ms12345678`.
  3. Publish SPF for each domain including Microsoft, for example `v=spf1 include:spf.protection.outlook.com -all`.
  4. Block or re-configure any system that sends as domains you do not own.
  5. If the block remains after cleanup, open a support case with Microsoft.

Checks that apply

  • TXT Lookup Confirms the Microsoft verification TXT record is published for each domain you add.
  • SPF Check Checks that every sending domain authorizes Microsoft 365 in SPF.
  • MX Lookup Shows whether the domain’s MX already points to Exchange Online, a sign it belongs in the tenant.
See the example diagnosed

The classification runs without network lookups and we never store your paste.

FAQ

Do I need a separate tenant for every domain I send as?
No. One tenant can hold many accepted domains; they just need to be added and verified.
Does this block my registered domains too?
Normally only messages from unregistered domains are refused, but a serious case can affect the tenant more widely.

Microsoft 365 delist portal Outlook.com SNDS

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