Same story. Ran a set of about 20 postfix mail servers for my company. Prided myself on being proactive, setting everything up correctly. Using things like DANE even before they were mandatory but the trouble was always random blocking which was outlook.com 90% of the time and occasionally yahoo and Deutsch telekom. At least the latter provided contact details and were helpful getting things unblocked but Microsoft had horrific support. You are blocked, we won't or can't tell you why, nothing in the mail management system shows you any problems so we can't tell you what to fix, please see this 15 year old guide as to how to setup email properly and we will tweak the setting a bit to let your mail through until the next time we decide to block it.

So frustrating because they have no legal obligation to accept our emails but our product relies on them being delivered.

I gave up and moved everyone over to SES instead.