You're running dovecot, therefor did not follow the How-to to the letter!
In the how-to they tell you to install courier and saslauthd, not dovecot.
(see page 4:
http://www.howtoforge.com/perfect-se...ispconfig-3-p4 )
i suggest you do the how-to again, and follow the instructions exactly as they are described in the how-to, nothing else (unless you know what you're doing)