Isn't this rather neat? Okay, terribly malicious, but kind of neat. Admit it! So all a cracker has to do now is get a mac or a linux / unix machine, install an SMTP server on it, configure it properly, which is, of course, improperly, and then they send an e-mail from the guy they want to attack to as many addresses as they can, and then all of the bounces hit the target. Now get four or five mail servers you can do this from in colo.

Look, a full mailbox doesn't bother a user much. But you fill the mail queue on a server, and that server becomes super slow. You can do some serious damage with this one. I'm impressed.

Administrators, double check your servers. Make sure your server can't be used to do this by accident. Because now there are people out there looking for open relays in the hopes that they're configured this way, and when they find one they'll use it.