Microsoft regularly sends legitimate emails from Microsoft to my spam folder.
Even with a six-figure email spend and weeks of troubleshooting the best response we could get from our mail provider was that they were having problems getting traction with Microsoft on the issue.
One of my main emails is still on a "free" outlook.com hosted with a personal domain that I never shifted to paid 365. I've also got an MTA server (mailu) of my own that I've been testing with... my own email under outlook.com is literally the only one of the MS systems I can't seem to deliver to, the rest work fine. Same for google.com for that matter... kinda wild.