We have developed a web app using PHP 5.0 and MySQL 5.0. It runs on Apache on a Linux Redhat. It has a feature by which, the app sends email to the user's registered email address using mail() function.
Emails (sent from our server) are received only for GMail or Hotmail email account. It is not receiving for Yahoo or Rediffmail other email account.
Any idea of where to start debugging this?