Routing the delivery of remote mail with hubbed_hosts on Exim

http://manned.org/hubbed_hosts

 

/etc/exim4/hubbed_hosts
       is an optional file containing a list of route_data records which can be
       used to override or augment MX information from the DNS. This is
       particularly useful for mail hubs which are highest-priority MX for a
       domain in the DNS but are not final destination of the messages, passing
       them on to a host which is not publicly reachable, or to temporarily fix
       mail routing in case of broken DNS setups.

       The file should contain key-value pairs of domain pattern and route data
       of the form

       domain: host-list options
       dict.ref.example:  mail-1.ref.example:mail-2.ref.example
       foo.example: internal.mail.example.com
       bar.example: 192.168.183.3

       which will cause mail for foo.example to be sent to the host
       internal.mail.example (IP address derived from A record only), and mailto
       bar.example to be sent to 192.168.183.3.

       See spec.txt chapter 20.3 through 20.7 for a more detailed explanation of
       host list format and available options.