Email validation for PHP

Here is the function to validate an email address with PHP.
It will check whether the domain has a valid mx record or not.

function validate_email($email)
{

// regular expression validation for email address
$regexp = "^([_a-z0-9-]+)(\.[_a-z0-9-]+)*@([a-z0-9-]+)(\.[a-z0-9-]+)*(\.[a-z]{2,4})$";

$valid = 0;

// Validate the syntax
if (eregi($regexp, $email))
{
list($username,$domaintld) = split("@",$email);
// Validate the domain
if (getmxrr($domaintld,$mxrecords))
$valid = 1;
} else {
$valid = 0;
}

return $valid;

}