I am Running Exchange Server 2003 SP2. I am using a backup mx service my ISP offers, please see my mx records below. The problem I am having is spammers are sending their email through the backup mx server instead of directly to my server, so when the spam finally ends up getting forwarded from the backup mx server to my email server it seems that both RBL checks and domain validation checks are failing on these spam messages. For now I have blocked all email from the backup mx server by using connection filtering in exchange and blocking the mx servers ip address. What can I do to defeat this work around that the spammers are using to bypass some of my spam filtering in exchange?
mydomain.com MX preference = 20, mail exchanger = relay1.sea.eschelon.com
mydomain.com MX preference = 10, mail exchanger = mail.mydomain.com