Let say we already have a functional SMTP server that is responsible for routing all mail for a given domain. Now we add a new server into this network that must be able to send its own emails, for example for administrative purposes (alerts, crontab, logs, emails generated by the applications, etc …). This new MTA doesn’t have to manage mail for users.
The clean way to do this is to configure our exim daemon to send mail via the “official” SMTP server. For this, just modify the /etc/exim4/update-exim4.conf.conf file like this:
dc_eximconfig_configtype='satellite'
dc_other_hostnames=''
dc_local_interfaces='127.0.0.1 ; ::1'
dc_readhost='mydebian.mydomain.com'
dc_relay_domains=''
dc_minimaldns='false'
dc_relay_nets=''
dc_smarthost='myrelay.mydomain.com::25'
CFILEMODE='644'
dc_use_split_config='false'
dc_hide_mailname='true'
dc_mailname_in_oh='true'
dc_localdelivery='mail_spool'
Now the host mydebian.mydomain.com can send emails using myrelay.mydomain.com‘s MTA.