If you think of writing groupware and have not checked out the capabilities of citadel, you should check the documentation. The documentation about how you can make use of the software from a programming standpoint is out if this world!
This software has most of what you would want in groupware (hint, user driven additions). In addition, the project is responsive to additions that will add to the project and are extremely helpful.
Easy to set up. Easy to use, Most importantly, features that are well thought out and implemented.
Love this project!
Thanks to all the Citadel folks for this great package!
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.