[Dovecot] Sizing MTA servers

Hi All,

This is probably a bit off-topic but does anyone have any idea about sizing MTA servers.   We have about 200,000 emails/hr  incoming and outgoing.  I am intending using Exim and Spamassassin on each MTA.  How many servers using recent hardware would I need to cope with this mail throughput?  What is more important on the servers, CPU or RAM?  Should I have mail going both directions on each server or dedicate a pair each for incoming and outgoing mail>

Thanks for any feedback.

Murray