Updated: 22 October 2026 · Applies to: Outlook.com / Hotmail / Microsoft 365 mail delivery
Before trying to get an IP removed from Microsoft's block list, confirm it is actually blocked rather than just landing in spam, since the fix for each is different.
Common reasons Microsoft blocks a sending IP
- A sudden, large change in your sending volume.
- Misconfigured DNS for the sending domain.
- A high bounce rate.
- The IP already has a poor reputation.
- Recipients marking your mail as spam.
- The IP has sent high volumes of spam, whether from you or a previous user of that IP.
Confirm it is actually blocked
A message that was delivered but landed in the spam folder is not the same as being blocked; that happens for other reasons and has a different fix. A genuine block shows an explicit non-delivery report (NDR) from Microsoft stating the IP is blocked, not just "delivered but not in the inbox."
Find the problem
Check your mail server logs for the rejection. Microsoft's block responses typically mention:
protection.outlook.com http://mail.live.com/mail/troubleshooting.aspx list (S4205)
Find the first time this appears in your logs, then check for anything unusual in the 24 to 48 hours before it: a compromised account suddenly sending spam, or an unusual number of failed login attempts are common causes.
Check your sender reputation
Use any IP reputation lookup tool to check your sending IP's score; a low score (commonly cited as below 85 on a 0 to 100 scale) can itself be enough to trigger blocking even without an obvious single incident.
Check your authentication records
Confirm your PTR (reverse DNS), SPF, DKIM and DMARC records are all correctly set up for the sending domain. Missing or misconfigured authentication is one of the more common root causes behind IP reputation problems with Microsoft specifically.
Submit the delisting request
Once you have fixed the underlying cause, submit Microsoft's own sender support/delisting form, including a copy of the bounce message and its headers. Processing is commonly reported to take around 24 to 48 hours; you can send mail normally again once the IP is delisted.
Reduce the chance of this happening again
Sign up for Microsoft's Junk Mail Reporting Program (JMRP), which notifies you when a recipient marks your mail as spam, so you can react before it becomes a full block.
