If you just start [...] trying to deny service to someone's Web site just because they sent you UCE, you're going to inconvenience a lot more people than the marketer.

Additionally, it might well be the case that the owner of the domain may not have realised their mail server is being used for spamming. Hence a quick note to the responsible person of this domain might wake them up and actually get them to secure their mailserver - which would show that they are indeed responsible...

Sometimes intruders are clever enough that the admin of the victim mailserver doesn't see the intrusion. And if someone tells you his/her system has never been hacked, s/he is either downright lying, doesn't have much of a clue, or his/her system is no older than maybe a week. The secret is to build your security so that you can easily detect intrusions...

Update: Aristotle is right, this is not the case here. Once I had come down this far, I based this post on the post above, not the top one. My bad...

--cs

There are nights when the wolves are silent and only the moon howls. - George Carlin