The SMTP proxy provides a relay for email. It determines access based on source and destination addresses. The only content filtering that the proxy performs is based on the source and destination values for the mail itself. The source (the sender's address) is compared to the list of spam domains; if the address matches any specifiec spam domain, the packet gets dropped. The destination (the recipients's address) is compared with the local domain to see if relaying is being attempted. If relaying is allowed, the packet gets passed through, if not, the packet gets dropped.
Point the MX record for the domain to the proxy for mail to be processed properly.
SMTP connection will then be made to the proxy, rather than to the actual SMTP server.