# 550 5.7.750: client blocked from sending from unregistered domain

URL: https://iblockspam.com/smtp-errors/550-5-7-750
Last updated: 2026-10-09

Code: 550 5.7.750 (permanent failure, the message bounced)
Provider: Microsoft 365 / Outlook.com

Microsoft 365 blocked outbound mail from your organization because a large volume was sent from a domain that isn't an accepted, validated domain in your tenant.

What the bounce says:

```
5.7.750 Client blocked from sending from unregistered domain
```
The complete NDR text as given in Microsoft's article. Its NDR reference table words it as "Service unavailable. Client blocked from sending from unregistered domains".

Microsoft says this NDR is typically generated when a large volume of email is sent from a domain that isn't an accepted domain in your organization. Microsoft 365 only allows a certain amount of mail to be sent from an unregistered (unaccepted) domain. Its NDR reference describes the trigger as a suspicious number of messages from unprovisioned domains coming from the tenant.

The NDR goes to your own users. It's Microsoft 365 refusing to send your outbound mail, not a recipient refusing it. Common ways to get here: an application, scanner, website or on-premises server relays mail through Microsoft 365 with a From address in a domain you never added; a domain was removed from the tenant but something still sends as it; or a connector is set up wrongly, which Microsoft names as a less common cause.

There's also a security angle. Microsoft warns that attackers sometimes create new inbound connectors in organizations to send spam, so check for connectors you don't recognize and for compromised accounts before you ask for the block to be lifted.

Only an email admin can fix this. Microsoft's most common solution is to add and validate every domain you send from. Where mail comes from an on-premises server or another service through a connector, Microsoft recommends a certificate-based outbound connector whose certificate domain is an accepted, validated domain. After you've addressed the cause, contact Microsoft Support to have the block removed.

## Common causes

- Mail sent through Microsoft 365 with a From domain that isn't an accepted, validated domain in the tenant.
- A device, application or on-premises server relaying through a connector with a domain you never added.
- An incorrectly configured connector.
- An attacker-created inbound connector, or a compromised account, used to send spam through the tenant.

## How to fix it (sender)

- Add and validate every domain you send from in the Microsoft 365 admin center. Microsoft calls this the most common solution.
- For relays from on-premises servers or services, use a certificate-based outbound connector whose certificate domain is an accepted, validated domain.
- Review every connector and remove ones you don't recognize. Check for compromised accounts at the same time.
- When the cause is fixed, contact Microsoft Support and ask for the block to be removed.

## How to fix it (recipient or admin)

- No action needed by recipients: the block is in the sender's Microsoft 365 tenant.

**Q: What counts as an unregistered domain in Microsoft 365?**
A domain that isn't an accepted domain in your tenant. Microsoft says Microsoft 365 only allows a certain amount of mail from such domains, so adding and validating every domain you send from is the most common fix for 5.7.750.

**Q: Does the 5.7.750 block lift by itself once the domain is added?**
Don't count on it. Microsoft's guidance is to address the underlying cause first, then contact Microsoft Support to have the block removed. Include what you changed, such as the domains you added and any connectors you removed.

Sources:
- Fix NDR error codes 550 5.7.703, 550 5.7.705, 550 5.7.708, and 550 5.7.750 (Microsoft Learn): https://learn.microsoft.com/en-us/troubleshoot/exchange/email-delivery/ndr/fix-error-codes-550-5-7-703-550-5-7-705-550-5-7-708-550-5-7-750
- Email nondelivery reports (NDRs) and SMTP errors in Exchange Online (Microsoft Learn): https://learn.microsoft.com/en-us/troubleshoot/exchange/email-delivery/ndr/non-delivery-reports-in-exchange-online

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