Any way to prevent a recurring mail loop in Mail Enable

[BigWhiteDog]

We have a client who, while setting up the company wide mail system, accidentally pointed 4 mail groups at each other then sent out a test mail setting up a redundant recurring loop generating over 22,000 duplicate emails and crashing the server! Is there anyway (other than shooting the client) to prevent crossing email groups. We have Plesk 7.5 reloaded w/ MailEnable standard and Horde Imp webmail.