"3. Mail Exchanger existence checks

Again, due to the robustness principle, just because a host does not define MX records does not mean they can’t accept mail. Mail servers will often fall-back to A records to try and deliver mail. That’s why we go one step further than just a DNS query, we ping the Mail Exchanger to make sure that it actually exists."

Plenty of boxes don't respond to pings (icmp). Can I assume you're doing a tcp scan on mail ports?