Well, I did as advised, deleted the relay_recipients.db file deleted all versions of my relay_recipients file to ensure I didn't use the wrong one, then added the 8200 email addresses to a new relay_recipients file, ran "postmap /etc/postfix/relay_recipents" restarted postfix and for good measure even restarted the server.
After a few days of testing I can confirm that <firstname.lastname@example.org> still comes through, but oddly, it is only @mydomain.com that get through so it is filtering against the domain name, just not the full email address.
"Necessity, who is the mother of invention." Plato, The Republic
Greek author & philosopher in Athens (427 BC - 347 BC)
"Invention, who is the mother of all necessity." Filfish