Mails sent to mail.protection.outlook.com are not being received

Solution 1:

We had a very similar problem a while ago. The reason, in our case, was threefold:

1. Microsoft rely on their own spam blacklist, which is not included in the normal blacklist checks, such as MX Toolbox.
2. Microsoft do not use standard methods of rejecting spam mail: instead, they accept it and then dump it into a black hole, so normal diagnostics will not work properly.
3. Microsoft have a get off our spam blocklist form, but if you open it in Firefox you get a Page Not Found error (it does work in Chromium).

Fill in that form, and follow all instructions. At the end, you have an option to sign up to alerts from their own spam blacklist.