Restrict external hosts

[Vernon A. Fort]

I have a setup which we use an external mail filtering service and need 
to limit/restrict external client access.  Meaning the MX for the domain 
points to the filtering service and they relay checked email.  I need to 
limit access to just these network blocks but also allow sasl 
authenticated as well as the internal network.

I also do not want to blindly trust this service so i would like to 
check the IP address as well as ensuring the recipient is for my domain.

can someone point me to an example or man page.  I cannot seem to find 
anything related to limiting inbound smtp clients/servers.

Vernon