Depend on what features you need. It should be easy to use iRedMail as a anti-spam gateway. the most important thing is anti-spam, you should config postfix and other components to query your exist mail server, e.g. exchange, postfix or whatever, so that iRedMail can reject emails which sent to non-exist users, it will save your server resource.

It would be great if someone can share his tutorial, or is anyone interested in developing an anti-spam edition based on iRedMail?