MTAs should be checking the *envelope* header regarding relaying, not the RFC822 From: header.
Also, most MTAs can (and perhaps should) be set to relay mail coming from a certain set of IP addresses. Your should be set to allow relay of mail from the IP address of your OACS4 server. This ought to solve this "relaying denied" problem safely and quickly.
I'll let others deal with the first two parts 😊