550 5.7.750: Blocked from sending from unregistered domains (Microsoft 365)
Your Microsoft 365 organization sent a lot of mail from domains that aren't added and verified in the tenant, so Microsoft blocked it.
Blocked The address is probably fine. The problem is how the mail was sent.
What the bounce message looks like
The wording changes from one provider to another. These are typical examples:
550 5.7.750 Service unavailable. Client blocked from sending from unregistered domains.Common causes
- Sending from a domain that hasn't been added to Microsoft 365
- A relay connector passing mail for other domains
How to fix it if you sent the email
- Tell your IT admin.
If you manage the receiving or sending mail system
- Add and verify every domain you send from in Microsoft 365, then contact Microsoft Support to remove the block.
Free tool: paste the full bounce into the bounce message decoder to get a plain-English reading of every code in it.
Questions people ask
- What does 550 5.7.750 mean?
- Your Microsoft 365 organization sent a lot of mail from domains that aren't added and verified in the tenant, so Microsoft blocked it.
- Is 550 5.7.750 a hard or soft bounce?
- Blocked. The address is probably fine. The problem is how the mail was sent.
- Should I remove the address after a 5.7.750 bounce?
- No. Fix how the mail is sent; the address itself is probably fine.