I had 10/10 during years, yet outlook would block emails from one of my domains - even if I was replying to an incoming email!! Microsoft seems to have lots of

I had 10/10 during years, yet outlook would block emails from one of my domains - even if I was replying to an incoming email!! Microsoft seems to have lots of secret rules, blocked IPs and other stuff that nobody's really figured out. I did manage to contact them one, I got white-listed, and email went through. But only for a few months, and then it went back to being silently dropped again. It was only for a particular domain. Other domains on the same host has no issues. I had to move the client in question to Fastmail, since there seemed to be no way to figure out how to please MS, and my client had lost several big sales due to documentation being silently dropped. Fastmail deals with it somehow. I guess they have some human contact with some influence in MS anyway, being large as they are.