To enable reverse DNS lookups in Exim, you would edit your exim configuration file (usually /etc/exim.conf) and add the following towards the top of the file:
host_lookup = 0.0.0.0/0
This will perform a reverse lookup for all incoming connections. Be sure that this is placed toward the top of the file in the "Main" section, and not lower down in one of the ACL, Router, or Transport sections.
If you are running cpanel, you cannot 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.