function is_valid_local_part($local) {
    // I'm not entirely sure about this regex, but it seems to fit was RFC2822 specifies.
    return preg_match("@^[-a-z0-9!#\$%&'*+/=?^_`{|}~]+(\.[-a-z0-9!#\$%&'*+/=?^_`{|}~]+)*\$@", $local);
}
 
function is_well_formed_email_address($email) {
    // We ignore legacy addresses and local addresses.
    $parts = explode('@', $email, 2);
    if (count($parts) != 2) {
        return false;
    }
    list($local, $host) = $parts;
    return is_valid_local_part($local) && is_well_formed_hostname($host);
}