550 5.7.606 is a permanent SMTP reply meaning access denied, banned sending ip (office 365). The 5.7.606–5.7.614 range is "Access denied, banned sender [IP]." Microsoft has flagged your sending IP for spam-like behaviour and blocks it at the edge.
What 550 5.7.606 means
The 5.7.606–5.7.614 range is "Access denied, banned sender [IP]." Microsoft has flagged your sending IP for spam-like behaviour and blocks it at the edge.
Common causes
Your sending IP built a poor reputation with Microsoft
Most commonSpam complaints, spam-trap hits, or a compromised account sending through your IP got it onto Microsoft's block list.
A compromised account or relay on your infrastructure
CommonA breached mailbox or open relay sent spam, and Microsoft banned the originating IP.
Shared-IP neighbour abuse
Less commonOn a shared sending IP, another sender's spam can get the whole IP banned.
How to fix 550 5.7.606
- 1
Stop the source of abuse first
Find and shut down whatever caused the bad reputation — secure compromised accounts, close any open relay, fix the spam source. Delisting without this is pointless.
- 2
Submit the Microsoft delisting request
Use the Microsoft 365 Anti-Spam IP Delist Portal (sender.office.com) to request removal of your IP. It's usually processed within 24–48 hours.
- 3
Lock down authentication and monitoring
Ensure SPF, DKIM, and DMARC pass and monitor your domain so you catch a repeat before Microsoft re-bans you.
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.