Of all email services, delivery to MS hosted systems is absolutely the worst to deal with. It's completely opaque and almost impossible to resolve most of the time. They tend to direct you to paid channels to try to mitigate issues instead of actually responding to complaints for false positive flagging as spam.

For my small, personal email server, I just gave up on trying... I can deliver to Gmail and every other major email provider without issue, and even MS seems to be split into a couple different backing orgs.