If you can not compile courier on your system, you can use dovecot instead. As it can be sometimes tricky to compile courier on fedor and as there are no rpm packages available, we switched to dovecot in the latest installation guides (for Fedora 13):
http://www.howtoforge.com/perfect-se...ispconfig-3-p4
I guess that the setup should work for fedora 11 as well if you use the fedora 13 guide.