Permanent failure — do not retry
SMTP 550 5.7.750
blocked from sending from unregistered domains (Microsoft 365)
SMTP 550 5.7.750 means Microsoft 365 has blocked your tenant because it sent a large amount of mail from domains that are not registered as accepted domains in the tenant. The text reads "Client blocked from sending from unregistered domains." Every domain you send from must be added and verified.
How it appears
550 5.7.750 Service unavailable. Client blocked from sending from unregistered domains.
What causes it
- An app or server relays through Microsoft 365 using a From domain that is not an accepted domain.
- A connector is relaying mail for domains it should not.
- A compromised connector or account is being used to send spam.
How to fix it
In order — try the first one first.
- Add and verify every domain you send from as an accepted domain in the tenant.
- Use a certificate-based inbound connector for relayed mail.
- Review connectors for any you did not create, and remove them.
- Contact Microsoft Support once the cause is fixed.
Common questions
- What does SMTP 550 5.7.750 mean?
- SMTP 550 5.7.750 means Microsoft 365 has blocked your tenant because it sent a large amount of mail from domains that are not registered as accepted domains in the tenant. The text reads "Client blocked from sending from unregistered domains." Every domain you send from must be added and verified.
- Should I retry a 550 5.7.750 error?
- No. 550 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 550 5.7.750?
- Add and verify every domain you send from as an accepted domain in the tenant.
Other 550 replies
- 550 5.1.1 — mailbox does not exist
- 550 5.7.1 — message refused by policy
- 550 5.4.1 — recipient address rejected, access denied
- 550 5.2.1 — mailbox disabled, not accepting messages
- 550 — mailbox unavailable
- 550 5.1.0 — address rejected, other address status
- 550 5.1.10 — recipient not found (Microsoft 365)
- 550 5.1.8 — bad outbound sender (Microsoft 365)
- 550 5.4.5 — daily sending limit exceeded (Gmail)
- 550 4.4.7 — message expired in the queue
- 550 5.7.25 — no PTR record for sending IP (Gmail)
- 550 5.7.26 — blocked, sender unauthenticated (Gmail)
- 550 5.7.27 — SPF authentication failed (Gmail)
- 550 5.7.28 — unusual rate of unsolicited mail (Gmail)
- 550 5.7.23 — SPF violation (Microsoft 365)
- 550 5.7.124 — sender not in allowed senders list (Microsoft 365)
- 550 5.7.509 — DMARC failed, policy reject (Microsoft 365)
- 550 5.7.606-649 — banned sending IP (Microsoft)
- 550 5.7.705 — tenant has exceeded threshold (Microsoft 365)
- 550 5.7.708 — traffic not accepted from this IP (Microsoft 365)
- 550 5.7.515 — authentication required for high-volume senders (Outlook.com)
- 550 5.7.1 S3140 — IP range on Outlook.com block list
Last reviewed 2026-09-11