Hi, earlier on i created a similar thread with the issue of php mail() getting filtered and going to the junk folder. I didnt manage to get an answer from the thread but after weeks of trying, these few things did the trick.
1. I created an MX entry for my domain. Just go to your cPanel and create an MX entry for your domain.
2. Add the proper headers to your php code. Remember to change the email address accordingly. And make sure that the email address actually exist on the server. You could create an email address in you cPanel.
PHP Code:
$headers = 'From: <admin@yoursite.com>' . "\r\n";
$headers .= '<admin@yoursite.com>' . "\r\n";
$headers .= 'MIME-Version: 1.0' . "\r\n";
$headers .= 'Content-Type: text/html; charset=ISO_8859-1' . "\r\n";
$headers .= 'Content-Transfer-Encoding: 8bit' . "\r\n";
$headers .= 'Return-Path: admin@yoursite.com' . "\r\n";
$mail = mail($to, $subject, $letter, $headers, "-fadmin@yoursite.com");
3. Under Email Authentication in your cPanel. Enable domainkeys and SPF.
Hope all these works out for you. =)