I'd suggest not wasting your money on a second e-mail server. Instead put your mail server on a separate interface off your firewall, and only allow the needed mail protocols to access it. Have the routing set up so that your internal users can still route to the mail server. Put on a server-wide antivirus package and implement policies about executable files.
For more info on this topic, visit these SearchSecurity.com resources:
Dig Deeper on Email and Messaging Threats-Information Security Threats
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