I use squirrelmail for my company.. it's easy to set up, looks pretty for the end user, and has online documentation for all of it's features.. so it keeps people off my back
There are only a couple of problems I've had with it.. we use an exchange server for email.. (it was there before I came
There aren't any plugins yet to support 'contact' public folders.. so all the contacts in the folders come up as emails.. and I couldn't get ldap to work..
Other then that though, it's great