I have been looking at high volume sources of inbound traffic.   My highest legitimate source IPs have sent up to 300 messages in an hour, and that even seems to exceed my highest volume spam source.   

I  have unhappy memories of a time that we were attacked with 700 messages in rapid succession from a single IP address.  The attack was not finished when we discovered the problem and blocked the IP address.    I want to prevent it from happening again.   SmarterMail is my current gateway, but it cannot track or throttle this type of attack.   It limits the number of messages per session, but allows a sender to immediately reconnect on a new session, which is exactly what this attacker was doing.

A quick web query says that this can be done with PostFix, so I could go that route.