Case studye LTM for Mail servers

For general smtp implementation where smtp virtual server uses snat I would suggest doing this :

http://terminaltoowide.wordpress.com/2011/10/27/smtp-relay-irule/

since you would like to map original IP (or group of them) to single snat IP. Additionally I have an iRule which logs every single SNAT event for SMTP virtual servers , this way we can easily detect who is spamming our smtp servers :) , but a lot depends on your general setup in your network environment.