Spam....
I'm having fun right now watching spam propogate over the earth that uses a forged header that uses my domain. I've been getting messages from mail servers all over the world (last week was Russia, this week is Europe and now the US) saying that a message from me has been blocked and/or was found to contain a virus. Lucky for me I'm moving the domain and will disable the email accounts for a period of time to let the spammer play his game.