You can use gmail and connect the accounts via POP3 or IMAP.
6:10 pm on Mar 8, 2010 (gmt 0)
Squirrelmail has worked for me... but task demand is not that great (about 300 users). Not sure how it scales.
6:15 pm on Mar 8, 2010 (gmt 0)
I will throw a vote in for squirrelmail as well.
My demand is about the same as tangor's but it does handle it well.
I think squirrel has to be IMAP.
6:26 pm on Mar 10, 2010 (gmt 0)
Squirrelmail scales quite well to large multi-user environments. It is used by one of the largest ISPs in the Netherlands as their default webmail system. The main problem I see with it is the lack of development. They are in the 1.4 series now for several years, and the development of the promising 1.5 version with a more graphical GUI seems to be stopped. The good thing is that maintenance and security fixes for the 1.4 series are still available. I use Squirrelmail as the de facto webmail software on my systems. It works, but especially the HTML-mode viewer is not impressive. Also messages with HTML embedded in the main message don't display very well.
I have recently done a few installations with OpengroupWare (OGo) and Scalable OpengroupWare (SOGo). The functionality is impressive but it is a significant amount of work to get it working. OGo is a groupware application which offers also document management, task planning etc. SOGo is simpler in its functionality and focuses mainly on email, but it has a more appealing user interface than OGo and scales easily to several thousand users.