SquirrelMail

I can't tell you how it is from an administration standpoint, but my university uses SquirrelMail for its webmail installation. They recently upgraded to 1.4.9a which is an improvement over whatever old copy they were running. A lot of the complaints that I had about the older version (getting logged out when working on a draft, etc.) have been fixed.

Not bad for an open-source solution, but your users will love you if you use a more polished system like Gmail via Google Apps.

hi, im an administrator and manage some middle-scale servers with imap and smtp. we decided to use squirrel mail because of its great functionality, mass of useful plugins. squirrel has everything we need, if not, its easy to wrote an extension for it. i can highly recommend squirrelmail to everyone looking for a powerfull and customizable webmailer. great job done, folks! :)
our users love squirrelmail and prefer using it, even if they have some local mua installed on their local workstation.