What the receiver is telling you
The connecting IP is on Microsoft's block list and every Microsoft-hosted recipient will reject it. Functionally identical to 5.7.511 but keyed on the IP alone — the bounce text includes the delisting URL because Microsoft expects most of these to be shared-hosting ranges where the sender is not the offender. If you are on your own server or VPS, it is you; if you are on a shared relay, it is a neighbour, and you are stuck until the provider or Microsoft clears the range.
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
- Submit the IP at sender.office.com. Include what the IP sends and how much.
- Check the IP on the blacklist checker; if it is also on Spamhaus or Barracuda, deal with those too — Microsoft feeds from them.
- If the IP is a relay provider's, contact them. Only they can delist their range.
- Pause Microsoft-hosted recipients until it clears.
don't Do not keep retrying. The 5xx is final for that IP and retries add to the record.
Decode the whole bounce
Paste the complete failure notice — every code and the receiver's wording — and get each part explained together.
Common questions
Can I check whether an IP is banned by Microsoft before sending?
Not directly; Microsoft does not publish the list. A test send to an Outlook.com address is the practical check.
Why does the same IP work for Gmail and not Microsoft?
Different lists, different signals. Microsoft weights its own users' complaints, which can put an IP on its list that Google has no problem with.
Often seen alongside
Related reading
If this is the thing going wrong
The pages explain it. If you would rather it was simply fixed, that is the work I do.