Sender callouts is an existing feature in cPanel that tells Exim to hit a sender’s mail server to find out of the sender exists.  This tends to lead to some false-positives when aliases are used to send email.  If you want to keep callouts enabled but ensure that certain people are able to reach you, you can set up a whitelist for email addresses and domains that will not be checked.

Tutorial taken and condensed from http://forums.cpanel.net/showthread.php?t=45370

Important:  before you make changes, it’s a good idea to make a copy of your /etc/exim.conf file!

1.  Create the whitelist file to list the safe senders.  To allow an entire domain, use *@domain.com

touch /etc/whitelist_senders

2.  In WHM’s advanced exim configuration section, add this line anywhere in the first box:

addresslist whitelist_senders = wildlsearch;/etc/whitelist_senders

3.  Scroll down to the ACL section (the three boxes together) and look for these lines:

#sender verifications are required for all messages that are not sent to lists

require verify = sender/callout
accept domains = +local_domains
endpass

Change this to:

#sender verifications are required for all messages that are not sent to lists

deny
!verify   = sender/callout
!senders  = +whitelist_senders

accept domains = +local_domains
endpass

4.  In the same section, locate this part:

#!!# ACL that is used after the DATA command
check_message:
require verify = header_sender
accept

Change to:

#!!# ACL that is used after the DATA command
check_message:
deny
!verify   = header_sender
!senders  = +whitelist_senders
accept

Then save and exit.  When exim is done restarting, send a test message to and from your server to make sure mail is working properly.