The ISPmail tutorial is maintained since 2002. You may have followed former versions of the tutorial and now want to know how to upgrade your mail server to Squeeze properly. It is hard to provide exact instructions on what steps to go through. Here are some general hints:
Upgrade the operating system
First you should read the Debian Squeeze release notes before attempting to upgrade your system from Lenny to Squeeze.
Changing the Dovecot configuration
Squeeze uses a newer version of the Dovecot software. In the new version the "cmusieve" plugin has been renamed to "sieve" and needs to be renamed in the /etc/dovecot/dovecot.conf. Please consult /usr/share/doc/dovecot-common/README.Debian after upgrading Dovecot.
Roundcube instead of Squirrelmail (optional)
You can continue to use Squirrelmail as a webmail user interface. RoundCube is just a more modern interface so if you prefer something more fancy then you may want to read the page on using RoundCube.