What the receiver is telling you

The sending IP or domain is on Microsoft's own block list, separate from public blocklists like Spamhaus. Microsoft maintains it from its users' spam reports and its own detection. Mail from a banned sender is rejected at connection for every Microsoft-hosted recipient — Outlook.com, Hotmail, and every Microsoft 365 tenant that has not overridden it. For a cold email operation this is the Microsoft equivalent of Gmail's 5.7.28.

Why a policy block must not be treated as a bad address

Sending tools file every 5xx under "bounced", and the natural reflex is to scrub the address. For a policy block that is exactly wrong. The recipient exists and would have received the mail; the receiver refused it because of your domain, IP, authentication or content. Scrubbing throws away a lead and leaves the cause in place, so the next send produces the same block against the next thousand addresses. The right response is to stop sending from the affected domain, find what the receiver named, fix it, and resume at low volume.

What to do

1. Request delisting at sender.office.com. Give the IP, the domain, and an honest description of the mail.
2. Find out why you were listed before it is lifted: spam-complaint rate, bounce rate, or a compromised inbox. Delisting without fixing the cause lasts a day or two.
3. Pause sends to Microsoft-hosted domains until the delisting is confirmed — you can identify them with the MX checker.

don't Do not switch IPs or domains to evade the ban. Microsoft correlates, and the new ones join the list faster.

Decode the whole bounce

Paste the complete failure notice — every code and the receiver's wording — and get each part explained together.

Common questions