A Smart Host is a SMTP server that will accept mail from another server and then deliver the mail for that server.  For example, server1 is setup to use server2 as a smart host. Anytime someone sends e-mail on server1 it is automatically relayed to server2 regardless of the MX entrys for the domain. Server2 then accepts this message, looks up the proper delivery host/IP and attempts to deliver the message to that host.

To configure a smart host, create /etc/exim.conf.local and add the following lines. Be sure to change to the hostname or ip of the smart host server:

driver = manualroute
domains = !+local_domains
transport = remote_smtp
route_list = * host.name.of.smart.host.server

Next run /scripts/buildeximconf

Then run /scripts/restartsrv_exim

Thats it, watch the logs for a bit to make sure it’s working!