I'm self-hosting my mails. Microsoft expects you to send enough mails to consider your mails not as spam. One thing Fastmail does that people who self-host can't is: sending enough mails.

I don't know what other measure they may take but this alone probably goes a long way.

And yes, this is fucking frustrating. I'm not going to send thousands of mails per months just for the sake of it and don't know how I should do that without spamming. This is so fucked up.