[exim] Complete mail management system?

Hi,

I was wondering if anyone know of a complete mail management
system (preferably using Exim), which would allow for management
of users, mail domains, junk filtering and such through the web.

It is important that the system is open source and that people
can contribute features and fix bugs.

Alternatively, maybe some of you know of established companies
that offer large-scale mail hosting, with good junk filtering and
stuff like that?

Thanks,

Morten