IMAP folders showing no messages in Office 365 Outlook

I'm new to Dovecot. Using Dovecot IMAP with Postfix, everything works fine using Thunderbird. But with the same setup, no messages show in IMAP folders for user accounts created in Outlook, accessing exactly the same server as the accounts in Thunderbird.

Dovecot version 2.3.7.1.

Is there any configuration required that is special to support Office 365 Outlook clients? How to debug this?

Andrew