I am new here so if this has been covered before, I apologise.
What I am desperate to know is this:
WHY is there not a system whereby spam can be bounced back to the senders?
This would cause the bastards so much grief that they would eventually give up their evil practice.
Welcome. 
COMOOD is focused on building trust on the internet.
Two wrongs don’t make a right. 
Comodo’s Anti-Spam is due to have a new release in May which is meant to fix a number of bugs. Comodo Anti-spam uses Challenge Response system which automatically bounces back with a message asking for a passcode which is inbedded in an image. The message is also automatically reported to the domain owner.
There are some Antispams out there that do have the option of bouncing back the messages though spammers simply filter these bouncebacks in their own email system and simply delete them. By quarantining the message and alerting the domain owner is more of a pro-active approach.
The current version has a number of bugs so I’d wait for the next release in May. I personally use POPFile which is free. It’s a little complex to setup but works on a Baysian reclassification of messages and there is an option to quarantine spam messages and change the subject of a message.
While it’s a good idea to bounce back messages, by even bouncing the messages back, you are confirming that either your email address or your domain exists.
A great way to protect your emails is by digitally signing and encrypting them.
This can be done for free by running Comodo Secure Email…
Hope this points you in some sort of direction / and / or helps…
Eric
There is another problem with bouncing back spam - You may be bouncing to the wrong person.
Most SPAM fakes the email header and therefore who the email says it is from, is rarely the person it is actually from.