PHP Email Validator – Email MX DNS Record Check

Validating an email address is one of the hardest feats on the web. A valid email can be marketing
gold, but an invalid email address is dead weight. Not only does it require a CPU-taxing PHP regular 
expression ("/^[A-z0-9\._-]+"."@" . "[A-z0-9][A-z0-9-]*". "(\.[A-z0-9_-]+)*"."\.([A-z]{2,6})$/"), 
the regular expression can be useless even after it's validated for format if the domain doesn't
exist. 

A regular expression simply wont do -- we need to think more low-level. What does email at any 
domain require? A DNS MX record. Well, PHP provides a great solution to validate that there's a MX 
record for the provided email address' domain.

Code Example:

<?php
function domain_exists($email, $record = 'MX'){
	list($user, $domain) = explode('@', $email);
	return checkdnsrr($domain, $record);
}
if(domain_exists('imran@phpmoot.com')) {
	echo('This MX records exists; I will accept this email as valid.');
}
else {
	echo('No MX record exists;  Invalid email.');
}
?>