Despite the age of the protocols, you can cheerfully connect a wide range of IMAP4 and POP3 clients to Exchange Online. If you do, you might need to consider how to handle calendar appointments, and if you want to use iCAL, you’ll need to make some adjustments with PowerShell.
https://office365itpros.com/2019/05/29/exchange-calendar-appointments-imap4-clients/
