I use citadel as an easy to install and use mail server solution. Being in a hurry installing an configuring mail services for customers that want to do basic administration themselves after setup I found citadel is a great choice as it provides a clean and easy installation (build and config scripts, debian packages) and both a shell- and a web-bases administration UI.
It's nice to have all those features as Calendaring, Address books and stuff but what I love most about it that you can hand an installation over to a non-technician with the good feeling that he can add and alter e-mail users himself. And: there is quite zero integration work to do: IMAP, POP, SSL etc. just works out of the box. No hacking around in cryptic config files. It just works.