April, you can pretty much count on the idea that the machines transmitting the spam to you are zombie client machines in people's homes on DSL lines. A significant number of them are probably in the Midwestern US, and in South Korea. They almost definitely don't run SMTP listeners, so if you attempt to send back to them, you're just going to be spinning your own server's wheels for nothing. In the unlikely event that they ARE running listeners, you're just abusing the bandwidth of people who are themselves hapless victims of internet malware. If you were to actually connect to a legitimate SMTP server, then it's almost an absolute certainty that this server's address was spoofed in what was sent to you, and you'll just be abusing yet another victim.
In other words, don't bother. You'll waste your own resources on slapping people that are just your fellow victims.
If you want to find better ways of handling spam, go here: http://www.chris-linfoot.net