Peformance - 4200 email users..

[tnats@]

Currently, I have 4200 email users using 2 simscan boxes to scan the mail, then send it to primary email box.

Question, what kind of horsepower would i need to host 4200 email users (and growing) using Plesk email. (qmail with virus/spam scanning).

I'm thinking a quad Xeon with 6 gig of ram.

Thanks for any advice,
Tom