These reports are troublesome indeed. Can anyone verify that actual bandwidth is being used, and it's not just reported (and not-really-used) bandwidth? If the bandwidth usage is real, the thought of outrageous cell phone bills - and who gets the blame - is scary, not to mention the effect of that traffic on the email server and network infrastructure.
Seems to me, though, that if the usage is real, the traffic might just be contained within the mail server; still not a good scenario, but "better", I guess, than wild network traffic.
From what I've read on the forum, some folks have fixed these issues by deleting and re-creating the email accounts; that's still a lot of work that should not be necessary, finding out which accounts are responsible etc. etc.