They actively make accessing office365 mail via IMAP a bad experience (slow, connection limit) in order to push people to their own clients. I've seen that a lot at work where we have some Java stuff searching and fetching specific mails from IMAP folders. That works very slow for office365 mailboxes and you'll be disconnected as soon as you open multiple connections.
@tsdh I think k9 works well enough with outlook/o365, so ig some UA spoofing could help maybe
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