[LUAU] EMail Gateway

[Randall Oshita]

Lets say I want to have an Email gateway to filter incoming and outgoing
SPAM and virus (Spamassassin, AmaVis, and some antivirus app) in front of my
MS Exchange box; what is needed, what must be configured and what are the
configurations?
I'm guessing the gateway has POSTFIX (or whatever)to relay the messages to
the Exchange box. Is there something I must do with DNS as well? Like MX
record for gateway AND(?) MS Exchange box? 
Looked through internet with no clear descriptions.
Randall