550 5.7.750 Service unavailable. Client blocked from sending from unregistered domains
Sample bounce line
550 5.7.750 Service unavailable. Client blocked from sending from unregistered domains
What it means
Exchange Online is blocking the tenant because a suspicious number of messages are being sent from domains that aren't provisioned (aren't accepted domains) in the tenant.
Why it happens
- A large volume of messages sent from a domain that isn't an accepted domain in the tenant.
- Less commonly, one or more incorrectly configured connectors.
- Attackers sometimes create new inbound connectors to send spam, and accounts may be compromised.
How to fix it
- Only an email admin in the organization can fix this.
- Add and validate all domains you use to send email in Microsoft 365 (the most common fix).
- Use a certificate-based outbound connector whose certificate domain is an accepted and validated domain.
- Look for unusual connectors and compromised accounts, and validate connectors.
- After fixing the underlying cause, contact Microsoft Support to have the block removed.
DNS record examples (replace example.com and the values with your own):
example.com. IN TXT "v=spf1 include:_spf.your-mail-provider.example ~all" ; one SPF record per domain; use your provider's real include value
Check your domain now
Many of these errors come from missing or broken SPF, DKIM, DMARC or reverse DNS records. Enter your domain to run the free check.
Related errors
No related errors listed yet.