My bouncing-spam experiment is officially over. I was getting over 1,000 spam messages per day before it started. I now receive less than 240 per day.
An important innovation was to reject all emails which contained my domain name, nealhere.com, in the "From:" field. This should ideally be done in the mail server, but I'm not exactly sure how to accomplish that, so I am doing it within my mail client.
This isn't helpful if you share a domain with other people - a gmail address, for example. But if you are the only person who receives email at your domain, it's a large improvement.
Three simple steps to reduce spam by 75%:3. Reject emails with your own domain in the "From:" field.