I don't see how else you could allow anon SMTP traffic from the Internet and not get taken advantage of eventually. If you don't use account auth and you don't\can't control the network layer either you're only hope would be to find a third party app that monitors incoming email messages and tries to verify the same originating IP\segment\ISP\domain isn't hammering out emails in your good name. I don't know if there is such a thing to be honest but I don't doubt there is someone out there who makes it. Even if there is software that will do this theres no way you can be protected from a botnet attack which most of them are now adays... Heck, now I'm curious if someone finds a solution to this.
