This is nothing user-facing. Microsoft will run that in the background, firewalling it off breaks it, so they'll have to act.

The emails are user facing. So if, say, the ISP were to detect Microsoft servers connecting and serve them back a mailbox with a single email in it instead of the user's real mailbox, then the user would open Outlook and see just a single message. Ideally non-threateningly titled "MICROSOFT HAS STOLEN YOUR PASSWORD" and containing clear instructions on how to switch back to direct IMAP.

That's a great idea and would be extremely easy to roll out. I hope people start doing that!

i can't wait to have to answer an email-captch for every imap connection in the future, just because Microsoft decided to do an Apple.