If the email server and client are on the same network, you could just tell the IPS to not ban that ip for very long or even at all. Having a computer on the network strickly for sniffing and various other IDS like tasks would probably be the easiest way to do it. If you are that worried about it have some sort of program monitoring the intra-lan and internet speeds to check for a steep rise, look in the IPS for something like a warning when u ban an IP.