Why the blacklist check finds nothing
This block is internal to Microsoft. Spamhaus, Barracuda and SpamCop know nothing about it, and a blacklist check will report the IP clean — entirely correctly. That is not a contradiction but the nature of the thing: large providers keep their own reputation data alongside the public lists.
The flip side is that there is no query that would let you check the status in advance. The block only becomes visible as a bounce.
Cause first, form second
Microsoft normally lists IPs after spam complaints, sudden volume jumps, or delivery attempts to many non-existent addresses. A delisting request without a fixed cause reliably leads to a new block — and repeated requests are treated with increasing reluctance.
- Confirm that you were actually sending. A compromised mailbox, an open relay or a web form without rate limiting are the usual suspects. A look at outbound volume in the mail log over recent days settles it quickly.
- Check reverse DNS. A PTR record that resolves back to the HELO name is effectively a prerequisite at Microsoft.
- Set up SPF, DKIM and DMARC completely. Without consistent authentication the reputation threshold is considerably lower.
- Clean your recipient lists. A high hard-bounce rate is one of the strongest negative signals there is.
The delisting
Only then fill in the form at sender.office.com. It is free and needs no Microsoft account. The reply is automated and usually arrives within hours. If the block persists, Microsoft refers you to support — at that point the IP is generally listed for sustained problems rather than a single incident.