But Microsoft can't even handle its own whitelist... and floats offensively absurd excuses for their incompetence.

I eventually caved in a couple of years ago and migrated our corp email system to MS (I'm the MD and we are a precious metal "partner").

Our MX records go to my trusty Exim n rspamd MTA on prem. If MS take the piss with subscription costs then, I'll simply move us elsewhere. If they all take the piss, I'll expand our Dovecot somewhat and worry about a calendaring app.

I've been doing email systems for about 30 years now and I think I have a handle on some of the issues involved.

Meanwhile for weeks at a time I got obvious spam and phising right in my inbox. It just didn't stop. Twenty mails per day. I was about to cancel my Office 365 subscription; and then from one day to the other it just stopped.