Our local ISP sends out emails like that to people, in case they arent aware of the large usage going on. The owner doesnt patrol unless the user is WAY over their limit for the month, but with the increasing amount of "zombies" (pc's that are compromised and used for ddos attacks, file serving, etc), he sends out letters as a courtesy.

"Did you know you did this much traffic this week?" Really? I didnt know that.. better look into it.

I'd say safely ignore their email.