Getting exim4 to relay to non standard ports
I spent the last several days wrestling with exim4 on debian. I know
nothing about SMTP servers, so I'm slowly and painfully learning how
to configure them. I have a custom SMTP server I'm actively
developing, and I need a workflow that allows me to develop on a stage
server and then eventually push those changes to a live, production
server. However, I only have one linode host to work with, and one IP
address. So I chose to have my custom server running on
stage.example.com:2501 and the production server example.com:2500.
Then I had exim4 relay to the appropriate server based on the
recipient domain.
In /etc/exim4/update-exim4.conf.conf file, make sure the following lines exist:
dc_local_domains='example.com'
dc_relay_domains='*.example.com;example.com'
Additionally, I needed to modify the remote_smtp transport
(/etc/exim4/conf.d/transport/30_exim4-config_remote_smtp). In the
"remote_smtp" section, immediately following "drive = smtp":
port = ${lookup{$domain}lsearch{/etc/exim4/domain_port_mapping}{$value}{25}}
Make sure you have the following in /etc/exim4/domain_port_mapping:
stage.example.com 2501
example.com 2500
One last addition is needed to make this work. We need to add a line
to the router that lets us know that it's okay to relay mail to
localhost. So, in /etc/exim4/conf.d/router/200_exim4-config_primary,
in the "dnslookup_relay_to_domains:" section, put this:
self = send
And that's it. As long as the servers are running on ports 2500 and
2501, this exim4 configuration will do what's necessary. Hooray for 5
days of pain!
