EXIM: Can't send email in local network

The default configuration for a Debian Sarge or Debian Etch system with Exim4 is to NOT allow MX records to point to internal IP addresses. This means, if you have a host inside your network and you want to send an email to someone inside your own network, you can't... at least by default.

The error message you are likely to see in your Exim log (/var/log/exim4/mainlog) after trying to send an email is:

In order to fix this default configuration (problem), you must make a change to the exim4 configuration. In Debian, when you install exim, it gives you the choice for split configuration or... not-split configuration. Depending on what you picked at that point, you will have to make this change in the corresponding place. I will show both files you have to change, so if you are unsure, just make the change in both places.

The setting we want to change is 'ignore_target_hosts' in the dnslookup section. By default in Debian this value is set to all private network IPs. We need to remove the private networks from this list. So the setting should be:

If you have a split configuration, the file you need to change is:

If you do not have split configuration, the file you need to change is:

Now if you've done any kind of customization to exim or if you aren't using Debian Sarge or Etch, these files might not be the ones you need to change. In that case, find out where the "router" configuration file(s) for Exim are, and then update the ignore_target_hosts in that file as shown above.

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