Hey, Postfix is configured with relayhost, it is also set to do recipient address verification (reject_unverified_recipient) in smtpd_recipient_restrictions. However, postfix by default won't relay any domains which are not listed as mydestination or relay_domains, to prevent "open relay". I need postfix to act as a "proxy" between internet and relayhost. So once it gets email from internet it will do recipient address verification callout on relayhost and if it's deliverable it will relay mail to relayhost. All domains are listed on relayhost server so address verification callout works with no issues, but it seems that postfix prevents any configuration to relay mail for domains which are not listed on itself in relay_domains, mydestination or transport_maps. Basically what it should do, is to be "open relay" but it will relay to internal relayhost (in case address verification callout is successfull) and not the internet, so no harm done. I know this can be done by synchronizing relay_domains or transport_maps from relayhost to this primary postfix, but i'd like to do address verification callout for this purpose.
You should be able to use milter-ahead to check reciepients http://www.milter.info/sendmail/milter-ahead/
