You could use Net_DNS2 to determine if the domain exists and if so, send the email on it's merry way.
include "Net/DNS2.php";
$r = new Net_DNS2_Resolver();
try {
$result = $r->query($domain, 'MX');
} catch(Net_DNS2_Exception $e) {
$result = null;
}
if ($result !== null) {
// send email...
}
Naturally, I'd suggest some level of caching so you aren't repeating lookups.