Noticed a menu item in Apple Mail called “bounce”:
“To discourage someone from sending you messages, you can reject or “bounce” messages from that person, as long as the return address is valid. Bouncing messages can be useful for rejecting mass mailings from companies whose mailing lists you’ve been added to after buying products.”
Cool, for the spam that does get through I can go on the offensive. As long as the sender’s email is valid, it will appear as though their message to me was delivered to an invalid address.