Web Mail to users

I am looking to offer web based e-mail to users who registered an account. I have my own server, so I have full root access to everything.

The web based e-mail need to allow a user to register for an account and have the server to set it up automatically. And it needs to have admin access for me to set it up, i.e. mailbox quota, ads on bottom of every e-mail, etc.

Does anyone know of one? Pref free & open source.