Are there any issues when changing the IMAP/POP3 server in Plesk from Dovecot to Courier?
In case Courier is installed instead of Dovecot consider the following:
This action will cause re-downloading of email messages by mail clients.
Courier does not support Sieve language and some server-side filtering rules will not be working.
It is required to update SSL settings for the Courier IMAP/POP3.