Mail.app duplicate emails thousand times and more

Hi Community,

we have a support mailbox based on an Exchange server 2010 and to which are some Mac users connected. Since one week we have the problem that particular emails get copied thousand times and much more. Today we have three emails copied more than 12.000 times. It seems that the problem is triggert by one of the Mail.app clients because if we close all Mail.app the copy stops. But it is not related to a single workstation, it differs from one workstation to another one. All other mailboxes on this server do not have this problem. We are helpless...

Does anyone have a clue what we can do to fix that problem?

We are using Mac OS from 10.7.x to 10.10.x

Thanks & Bye Tom