I have been researching for a while on MS-Exchange Replacement , with the following aims :
1) Something more stable
2) Easier to maintain
3) less expensive
I found the following
i) SCALIX http://www.scalix.com/
There is an Open Source version which gives full features with only 25 users that can have MS-Outlook Access to their inbox.
although that the Web interface it intutive, AJAX , and pretty fast
all features are accessible in the Open Source versions except the number of users limitation
SCALIX works on RedHat Enterprise Version , although that it is easy to make it work on other distros
there is support for AntiVirus, SPAM filtering and many other features.
the Community and Forum are active
ii) ZIMBRA http://www.zimbra.com/
I do not have much experience on it, as I did not try it extensively.
it is all time constrant, but for sure I will work on it and let all of you know.
iii) CITADEL http://www.citadel.org/doku.php
I just read about it, did not have the hands on experience, but I found this nice article regarding it.
it is in the pipeline as well, and will keep you posted
Nice Article http://www.linuxjournal.com/article/9357
Please if you have any other experience you can share with us, we love to hear for you