Block all incoming SMTP except from ip's of my spam filter service

I use two external spam filtering services. All MX records for the domains on my cpanel server point to the spam filtering service. I am getting spam from spammers that just email @mydomain.com and disregard the MX records. I'd like to modify exim so that it ONLY accepts emails sent to it from the ip's of my spam filtering services. It also needs to accept mail generated inside the server itself. I've already added those ip's into /etc/trustedmailhosts. All of the documentation I have found to do this refers to very old cpanel/exim installs. Can someone help me out? Thanks!