Assuming the double bounce is because they try to spam a non-existant mailbox. Yes it does. The email is never accepted to the server. When the spaming server contacts your server and says, hey I want to send an email from xxx to xxx, your server will respond, sorry I can't do that. I finally installed the smtp rules on my box here a few days ago--it took some custom work because I run ezmlm mailing lists and those aren't added to the accepted list by default.