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.

The Code

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

The Usage

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

You'll see that 'MX' is the default record check, though you could change that to 'A' if you want. This method is not bulletproof but the function does provide a compelling enough argument for an email existing or being a fraud. If a valid email address is important for your purposes, use this function. Try it for yourself!