if (!is_well_formed_email_address($email)) {
    echo "Email address is not well-formed.\n";
} else {
    list(, $host) = explode('@', $email, 2);
    if (can_host_receive_mail($host)) {
        echo "Host $host can receive mail.\n";
    } else {
        echo "Host $host can't receive mail.\n";
    }
}