try squirell mail. should do what you want. and its linux based.

Sanjay Aggarwal <isanjayaggarwal@...> wrote:
> hi guys,
>
> i am looking for a out-of-the-box webmail solution that i can
> integrate with an existing portal to offer email for a large
> number of users.
> what i have found so far:
> - zimbra: a little heavy but neat intarface
> - @mail: commercial, interface is not very rich
>
> does anyone have any other ideas? our current platform is
> java/mysql/linux based. i want all the users registered on our
> site to get an email address without having to register again.
>
> any pointers will be very useful.