> It is not stealing anything because you get a dialog asking you for permission to do it

That dialog talks about sync but notably does not mention credentials at all.

Surely this is instance where informed consent is needed, with full disclosure of what's going to happen.

Something along the lines of: "this means your IMAP username and password will be passed to Microsoft where we will store it indefinitely so we can regularly log into your IMAP server to sync your messages".

Of course, users are less likely to consent if you explain exactly what's going to happen...