Is 550 5.7.750 permanent or temporary?

550 5.7.750 is a permanent (5.x.x) failure — the server will not accept the message as-is, and retrying without changing anything won't help. You need to fix the underlying cause.

Is this my problem or the recipient's?

It depends on the cause — you send as a domain not added to the tenant is the most common. Work through the causes below to find which applies to you.

How do I check my sending setup?

Run a free EmailHealth check on your domain — it verifies SPF, DKIM, DMARC, blacklist status, and more in a couple of seconds, which covers the most common causes of delivery failures.

550 5.7.750 — Microsoft 365: blocked from sending from unregistered domains

550 5.7.750 is a permanent SMTP reply meaning microsoft 365: blocked from sending from unregistered domains. Microsoft saw a suspicious number of messages from domains that your tenant does not own. It blocks the tenant until those domains are added and verified.

What 550 5.7.750 means

Microsoft saw a suspicious number of messages from domains that your tenant does not own. It blocks the tenant until those domains are added and verified.

⚠️

This is a 5.x.x permanent failure — the message will keep bouncing until you fix the cause. Retrying the same message unchanged won't help.

Common causes

📨

You send as a domain not added to the tenant

Most common

Apps or connectors send with a From: domain that is not an accepted domain in Microsoft 365.

🔁

A relay connector passes mail for other domains

Common

An on-premises server or scanner relays mail for many domains through your tenant.

⚙️

A compromised account sends with forged From: addresses

Less common

Spam sent through the tenant used many random domains.

How to fix 550 5.7.750

1
Add and verify each sending domain
In the Microsoft 365 admin center, add every domain you send from and complete DNS verification.
2
Review connectors
Check outbound connectors and relay setups, and remove any that send for domains you do not own.
3
Contact support after the fix
When the domains are registered and the cause is fixed, ask Microsoft support to remove the block.

Confirm your domain is set up correctly

Most delivery failures — permanent or temporary — trace back to authentication or reputation. Run a free EmailHealth check to confirm SPF, DKIM, DMARC, blacklist status, and TLS are all healthy for your sending domain. If you send in volume, free daily monitoring will alert you before a small issue becomes a flood of bounces.

Microsoft 365: blocked from sending from unregistered domains