I have an Office 365 mailing account. I use it with an IMAP client and I have several performance issue. I suppose that this is caused by a throttling mechanism on your side. Please read the IMAP part of this: https://kb.wisc.edu/page.php?id=33784
I cannot mark as read five or more mails at a time, for example which is a huge problem when I want to check fast my inbox.
Could you help me please what can I do?