How about this-Generate multiple URL requests... (Score 1)

There's no way to 'strike back' via email with spoofing being so prevelant. For spam (caught by a filter lets say) that request resources from a URL or multiple URL's:

1) Generate 10-100X the number of requests to same URL or URL's

2) If large number of spam messages are sent, the reposnse will be equivalent to a Denial of Service attack. No meaningful response will be possible.

3) If the message was mistakenly identified as spam, then the response will be quite manageable and no harm done.

But...

4)A malicious spammer could send a large spam distribution with URLs that point to 'other' sites, thereby launching a DOS attack on anyone of their choosing.... hmmm.....

Oh. well... I thought I had something there...

Maybe keep an extensive list of 'No big repsonse load (DOS attack) here' sites???