SquirrelMail 2
http://www.squirrelmail.org

SquirrelMail is a lightweight, quick, and yet very feature packed webmail
application written in PHP.  Its modular design allows functionality to be added
to the core while keeping the entire app coherent and without bloat.  With the
ability for customizing the back end and the front end, this application
designed with the administrator and user in mind.  Easily and fully
customizable, yet at the same time, it's ready to use right out of the package.