This has been going on for years!
I run a server with just ONE email user – a friend of mine with a small business – who sends no more than a few emails a day in total. Despite this, about every six months Microsoft blocks the IP address, citing unspecified bad behaviour.
We've got DKIM and SPF set up. Searching the server logs reveals nothing suspicious. Microsoft flat-out refuses to tell us anything at all about why we've been blocked. No other spam blocklist shows us as blocked, and even Microsoft's own tools (JMRP and SNDS) give us the all-clear.
The only way to get unblocked is to follow a back-and-forth dance through the Microsoft help system (they always refuse to do anything at first, you have to send an annoyed reply) and wait a few days.
I suspect that the reason is that our IP address sends so few emails, it's being reclassified as a newly-in-service IP address. But Microsoft won't tell us anthing, so that's just a guess.
Long thread here: https://github.com/MicrosoftDocs/microsoft-365-docs/issues/592