I determined how to do what I'm looking for and thought I'd share the results, for those that might be looking...
1) Add valid recipient addresses to Proxy - SMTP - blacklist/whitelist - recipient whitelist
2) Add yourdomain.com to the SMTP - blacklist/whitelist - recipient blacklist
From what I understand and how the SMTP session is working, the smtpd_recipient_restrictions check_recipient_access btree:/etc/postfix/recipient_rules looks at the whitelist/blacklist you just established in order for a match in the RCPT TO
If there is a match, then it processes against the right side (OK or REJECT)
It does this in order from top down, with 'yourdomain.com' at the bottom
I tested this several ways and see that any non-valid email is being rejected immediately at the gateway (as it should), and valid email addresses get passed through to the next step in the smtpd_recipient_restrictions list.
I was concerned that the messages were just getting forwarded to my internal SMTP server, but I had a check by sending the GTUBE test (
http://spamassassin.apache.org/gtube/) to ensure the message was undergoing the amavis (greylist/SA) functions.
Now I'm onto user/group HTTP access
Great built-in logic and interface Endian!!!!