Enhanced SMTP status code · RFC 3463 · IANA
5.7.750 Service unavailable
The organisation sends a suspicious number of messages from domains not provisioned in Microsoft 365.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1).
- Subject: X.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
How 5.7.750 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Microsoft 365 / Exchange Online
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5.7.750
Service unavailable. Client blocked from sending from unregistered domains
The organisation sends a suspicious number of messages from domains not provisioned in Microsoft 365.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026