Table of Contents

Choose relay based on recipient domain

To choose an SMTP relay server based on the recipient domain.

/etc/postfix/main.cf 

#relayhost =
#Remove any relayhost entry
smtp_sender_dependent_authentication = yes
transport_maps = hash:/etc/postfix/transport

/etc/postfix/transport 

yahoo.com smtp:smtp.someotherserver.com

Run the following 

postmap /etc/postfix/transport
service postfix restart

Alternate setup

The example below is to define alternate transport for given exceptions. So the default will use what is configured in relayhost but defined domain names (in transport) will use an alternate. /etc/postfix/main.cf (keep relayhost as is) 

smtp_sender_dependent_authentication = yes
transport_maps = hash:/etc/postfix/transport

/etc/postfix/transport add below. This will cause only gmail to be delivered via localhost (and it's WAN IP) as SMTP. 

gmail.com smtp:

References