A small but useful addition to the scoring system of DNS Spy: support for the Null MX record.
Internet mail determines the address of a receiving server through the DNS, first by looking for an MX record and then by looking for an A/AAAA record as a fallback.
Unfortunately, this means that the A/AAAA record is taken to be mail server address even when that address does not accept mail.
The No Service MX RR, informally called "null MX", formalizes the existing mechanism by which a domain announces that it accepts no mail, without having to provide a mail server; this permits significant operational efficiencies.
Give it a try at the DNS Spy Scan page.