Microsoft is working towards disabling IMAP & POP when accessing Exchange.
Unfortunately the solution they are proposing requires a high level of administrative access to your Exchange instance and some pretty complicated code from Postie.
The recommended solution is either
a) Don’t use Office365 for your secret Postie email. Use a different provider that supports IMAP or POP.
b) Use an Office365 email, but forward all emails to a different provider that supports IMAP or POP.