1. Stop postfix and courier

systemctl stop postfix
systemctl stop courier-authdaemon
systemctl stop courier-imap
systemctl stop courier-imap-ssl
systemctl stop courier-pop
systemctl stop courier-pop-ssl

2. Install dovecot

apt-get install dovecot-core dovecot-imapd dovecot-lmtpd dovecot-mysql dovecot-pop3d dovecot-sieve
apt-get remove courier-base courier-imap courier-imap-ssl courier-pop courier-pop-ssl courier-authlib-userdb courier-authlib-mysql courier-authlib courier-authdaemon

This will remove the courier packages automatically

3. Login to ISPConfig, go to System > Server config > mail and set:

POP3/IMAP Daemon: dovecot Mailfilter Syntax: sieve

and click on save.

4. Create a remoting user, downlod the ispconfig courier to dovecot migration script, adjust the values in it and run the script.

php courier_to_dovecot.php

5. Run a ispconfig update and select to reconfigure services.

6. Reconfigure squirrelmail