1) Stop postfix and courier

/etc/init.d/postfix stop /etc/init.d/courier-authdaemon stop /etc/init.d/courier-imap stop /etc/init.d/courier-imap-ssl stop /etc/init.d/courier-pop stop /etc/init.d/courier-pop-ssl stop

2) Install dovecot

apt-get install dovecot-imapd dovecot-pop3d dovecot-common apt-get remove 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