What server for a mail server

[Olivier Nicole]

Hi,

I have a project where I should set-up a mail server for approximately
12000 users, 4000 virtual domains, 15000 messages per day, 700 MB
traffic per day.

The machine should run Postfix, courrier-imap and a web mail (probably
squirel), Apache 2, spamassassin, clamav, greylist

What type of machine should I target, in terms of CPU and RAM?

TIA,

Olivier