Method 1

1. Open WHM and click on Exim Configuration Editor
2. Click on the Switch to Advanced Mode button.
3. Scroll down until you see the section on ACL's. You'll see three boxes. In the middle box, locate the section that reads - See Section A:

Method 2 - Recommended

1. Login into SSH and su to root
2. pico /etc/exim.conf
3. Ctrl + W and type in: accept domains Notice the two spaces. Look for something like this below.

Section A:
accept domains = +local_domains
accept domains = +relay_domains
accept hosts = +relay_hosts
accept condition = ${perl{checkrelayhost}{$sender_host_address}}

Between:

accept domains = +local_domains

And :

accept domains = +relay_domains

Add the following:

endpass
message = unknown user
verify = recipient

Make sure each line lines up. You might need to add two spaces before each line. The final section will look like this:

accept domains = +local_domains
endpass
message = unknown user
verify = recipient
accept domains = +relay_domains
accept hosts = +relay_hosts
accept condition = ${perl{checkrelayhost}{$sender_host_address}}

Scroll all the way to the bottom and click on Save.