checkEmail.php

<?php
function checkEmail($email){
	// checks proper syntax
	if( !preg_match( "/^([a-zA-Z0-9])+([a-zA-Z0-9\._-])*@([a-zA-Z0-9_-])+([a-zA-Z0-9\._-]+)+$/", $email)){
		return false;
	}     
	// gets domain name
	list($username,$domain)=split('@',$email);
	// checks for if MX records in the DNS
	$mxhosts = array();
	if(!getmxrr($domain, $mxhosts)){
		// no mx records, ok to check domain
		if (!fsockopen($domain,25,$errno,$errstr,30)){
			return false;
		}else{
			return true;
		}
	}else{
		// mx records found
		foreach ($mxhosts as $host){
			if (fsockopen($host,25,$errno,$errstr,30)){
				return true;
			}
		}
		return false;
	}
}
?>