Exchange server and multiple emails

Im running Exchange Server 5.5 w/ latest SP. I have a user who is getting “one” email about 20 times a day. Im not sure who or what I should be looking at, a stop gap solution would be to set up a filter and block that email address. But I have seen this similar activity with a client from “compaq.com”. Im thinking its a problem on my Exchange server. How do I start troubleshooting this?