It probably will not make you feel any better, but there is plenty of evidence around that this is a Microsoft problem, and not restricted to attempts to use Fastmail. Here is a long thread with others struggling with the problem, some of whom suddenly saw the issue spontaneously resolve itself:
https://answers.microsoft.com/en-us/...0-204d675c9f10
I can recall struggling with Outlook IMAP issues some years ago, and managing to circumvent the issue without really understanding the underlying cause.