At the ISP I work at, I'm forever calling users because they have the Klez virus and it drives up the loads on the mail servers like mad. They have about 30-40mb of returned mail with attachments for just one user because of this virus. 30-40mb might not seem like much, but multiply that by thousands upon thousands of users and you can see what I mean. Not to mention the fact that it sends out mail so rapidly that we have big time mail latency!
