How to check users email address domain really exists?

We know how spam bothers much on contact pages of web application,even we use captcha ,there will be manually generated spam.
And we don’t know given email address is fake or real.

Good thing is that we can check is given email address fake or not with php .For this PHP has handy function that checks the email domain for real time exists.

Bad thing is that this function is not bundled until 5.3 on windows server.

checkdnsrr

function is_emaildomain($email)
 {
 return (bool)checkdnsrr(preg_replace('/^[^@]++@/', '', $email), 'MX');
 }