[Dovecot] Sizing MTA servers

[Murray Trainer]

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