Actually it's simpler than that- just configure your server to route everything to the ISP's server just as most mail clients do. The problem is that it would not take much time at all for the spammers to get around the block- all they need to do is get the spambot to route all the traffic through the ISP. The SMTP server address can be hacked from the registry. The net result is an exponential increase in the workload on the ISPs' servers with no significant reduction in spam volume.
If you run it through the ISP's mail server as client access you may run into mail quota checks, and it's going to interfere with forging the From: address and the headers.