In main.cf
relay_domains = $mydestination, hash:/etc/postfix/relay
transport_maps = hash:/etc/postfix/transport
In /etc/postfix/relay
example.com OK
example2.com OK
remember to postmap /etc/postfix/relay
In /etc/postfix/transport
example.com relay:192.168.1.10
example2.com relay:192.168.1.20
remember to postmap that also, then reload postfix.
Not sure about exact syntax but that should do, I have my conf in MySQL
Hope that helps / works