do you mean that you are looking to start something like hotmail... where users can signup and then check their mail via a web interface? or what?
cause if you want to offer it...which i think is what you mean ive used squireelmail and linux along with some other software to do virtual mail boxes ... which essentially would support welll a crap load of users .... if you want to see my setup email me at justin@debianbox.net
Go to sourceforge.net and search for "cpanel email" you'll get a hell lot of them.
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I have used hivemail and one proprietory developed by a local company here in Pakistan. If you are interested tell me the requirements in detail including the storage method needed (db/filesystem)