How do I configure postfix to deliver mail for specified domains to another host, and deliver the rest using MX records
Instructions work for Ubuntu, you may need to adjust for other distros.
Add the following to /etc/postfix/main.cf:
relay_domains = domain1.com, domain2.com
transport_maps = hash:/etc/postfix/transport
relay_recipient_maps = hash:/etc/postfix/relay_recipients
relay_transport = relay
Create the file /etc/postfix/transport:
domain1.com smtp:[mail.example.com]
domain2.com smtp:[mail.example.com]
Create the file /etc/postfix/relay_recipients:
@domain1.com x
@domain2.com x
Run these commands:
$ sudo postmap /etc/postfix/transport
$ sudo postmap /etc/postfix/relay_recipients
$ sudo /etc/init.d/postfix reload