I use postfix to send direct email. But, I use a relay to send email for specific senders. So I use "sender_dependent_relayhost_maps" :
relayhost = transport_maps = hash:/etc/postfix/transport smtp_sender_dependent_authentication = yes sender_dependent_relayhost_maps = hash:/etc/postfix/sender_relay
So when I send an email with sender email "email@example.com", postfix will use the relay.
But... I don't want to use the relay host for some recipient. ;)
I think I must use "transport_map" to catch the email before it pass through "sender_dependent_relayhost_maps" but I don't know how to do it...