There are tons of webmail software out there, and I'd like to get one that suits my needs.

Basically I need a webmail software that has these features:

Scalable
PHP written (so my developers can modify the code as needed)
Easily plugs in to my custom login system, or has good API to do so. I'm integrating this with my social networking site.
Gret features


Any suggestions would be greatly appreciated. :D

Can someone shed some light on this?

First choice, Squirrelmail (http://www.squirrelmail.org/)

SmarterMail is an excellent webmail system software (http://www.smartertools.com/Products/SmarterMail/Overview.aspx)