function  domain_exists ( $email ,  $record  =  'MX' ) {
     list ( $user ,  $domain )  =  split ( '@' ,  $email ) ;
     return  checkdnsrr ( $domain ,  $record ) ;
}

if(domain_exists('test@gmail.com')) {
     echo('This MX records exists; I will accept this email as valid.');
}
else {
     echo('No MX record exists;  Invalid email.');
}
?>