You may see the following error while sending mail via php script:-
2010-12-12 03:56:49 1Nr5AP-0007bm-Ef ** email@example.com R=checkspam2: Mail sent by user nobody being discarded due to sender restrictions in WHM->Tweak Settings
To fix this remove the following files from the server.
root@linux [/home]#rm /etc/eximmailtrap
root@linux [/home]#rm /etc/webspam
Issue should be fixed now.