Help! Bandwidth problems with an account.

Hi,

I posted this like 2 weeks ago in another forum but didn't get any views or replies ^^ Hoping for better luck here.

I have this customer who uses his account only for mail. Only this month he's used something like 4 GB bandwidth! I check webalizer and other stats and they all show almost zero bandwidth usage. Is there anyway I can check the mail bandwidth usage? Do you have any other suggestions how I can find out where this bandwidth is from?

Thanks.