Microsoft 365 5.7.750
Service unavailable
Microsoft 365 5.7.750 means: service unavailable. It is a permanent failure, so do not retry it unchanged. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Microsoft 365’s exact wording
Quoted from Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online, checked 2026-11-05. Placeholders are Microsoft 365’s own.
Provider's description
Service unavailable. Client blocked from sending from unregistered domains
Possible cause
A suspicious number of messages from unprovisioned domains is coming from this tenant.
Provider's advice
Add and validate any or all domains that you use to send email from Microsoft 365 or Office 365.
| Who sends it | Microsoft 365, Exchange Online and Outlook.com |
|---|---|
| Basic code | Microsoft 365 lists the enhanced code only |
| Enhanced code | 5.7.750: Microsoft 365-specific, not in the IANA registry. Its subject digit puts it under 5.7.0 other or undefined security status. |
| Permanent or temporary | Permanent (5xx) |
| Verifying the list prevents it | No |
In practice
550 5.7.750 Client blocked from sending from unregistered domains means a Microsoft 365 tenant sent a suspicious amount of mail from domains it has not added and verified. Add and verify every domain you send from in the Microsoft 365 admin center, then contact support to lift the block. This mostly affects new tenants and tenants with compromised accounts. Microsoft wants every domain used in the From address to be an accepted domain in the tenant, so mail sent as an unverified domain is treated as suspicious. Add the domain, verify it with the DNS record Microsoft gives you, and then ask support to lift the block. The block applies to the tenant.
What the sender should do
The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
Would verifying the list have prevented Microsoft 365 5.7.750?
No
This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Common questions
- What does Microsoft 365 5.7.750 mean?
- Microsoft 365 5.7.750 means: service unavailable. It is a permanent failure, so do not retry it unchanged. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
- Should I retry after Microsoft 365 5.7.750?
- No. It is a permanent (5xx) failure, so the same message to the same address will fail again. Fix the cause first. The fix is in the sending or receiving configuration named above, not in the list. Fix that, then resend.
- Would verifying my list have prevented Microsoft 365 5.7.750?
- No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.
Sources
- Microsoft Learn: Email non-delivery reports and SMTP errors in Exchange Online (checked 2026-11-05)
Registry and provider wording is quoted exactly; guidance is written by the SimpleVerifier Team.
SimpleVerifier Team · Last checked against the sources 2026-11-05