Login to WHM, Click on the “Tweak Settings” link under “Server Setup”. Within the mail section is a check box with the heading:
“Prevent the user ‘nobody’ from sending out mail to remote addresses (php and cgi scripts generally run as nobody if you are not using phpsuexec and suexec respectively.)”
Tick the check box and and apply the changes by clicking on the “Save” button at the bottom of the page.