Question
Is it better to choose Dovecot or Courier IMAP?
Answer
For IMAP and POP3 delivery, Courier IMAP or Dovecot can be used. Plesk recommends using Dovecot: "Dovecot is faster, more secure, and it consumes fewer server resources and provides support for server-side mail filtering rules".
Note: if you decide to change it later please check for potential issues before switching to Dovecot.
Additional information
How to install/change a local mail server in Plesk for Linux