For anyone running their own mail server, Microsoft has an authorization system to let you "authorize" your mail server IPs to talk with Microsoft's mail servers.

It's stupid, but not terribly difficult to set up and authorize.

Like most people who run their own mail, I have had repeated problems with Microsoft in particular, who treats small mail server operators like crap.

This post reminded me that I had gotten some email requesting me to "Re-attest Your Network Access". I went in, clicked a few things, and I'm good for another year or whatever.

https://substrate.office.com/ip-domain-management-snds/snds

https://www.litmus.com/blog/what-is-microsoft-snds-why-your-...