- For a generic and/or DirectAdmin powered server (s):

To enable reverse DNS lookup in Exim, you would edit your exim configuration file (usually /etc/exim.conf). The following directive is commented by default. You need to comment out this directive to enable rDNS lookup in Exim:

From:
#host_lookup = 0.0.0.0/0
To
host_lookup = 0.0.0.0/0

This will perform a reverse lookup for all incoming connections.

- cPanel/WHM powered servers:
If your server is cPanel/WHM powered, it is un-wise to edit exim.conf manually, as it will be overwritten. To enable this function, log into WHM and go to the advanced exim configuration editor. In this editor, the first box is where you will put the above configuration directive.