I wouldn't call it impossible...difficult, maybe. Do metrics exist on how many current installs still rely on the implicit MX? Is the abuse of the implicit MX causing more harm than the effort it would take legacy DNS admins to specify an MX?
If I understand your question, you're asking how many sites don't bother with an MX record, but count on the fallback to A to get their mail delivered. I have to say I don't know and don't know of anyone who has checked. I'm not sure that's its even possible to know without starting with a full knowledge of the mail-sending entities out there. Given that many entities allow for subdomain-level mail address (research.example.com, cs.example.edu), the number of extant domain names at some level of the hierarchy would only be a predictor. Possibly someone with a very large mail installation could run statistics to show how often they fell back; that wouldn't be perfect, but it would be somewhat useful. But I think the key question is actually different. Look at this text in RFC 2821: If one or more MX RRs are found for a given name, SMTP systems MUST NOT utilize any A RRs associated with that name unless they are located using the MX RRs; the "implicit MX" rule above applies only if there are no MX records present. If MX records are present, but none of them are usable, this situation MUST be reported as an error. If I put in an MX record pointing to a guaranteed non-present FQDN, someone complying with that text will throw an error rather than keep seeking for an A/AAAA. Is *that* useful? If so, then sink.arpa/1.0.0.257.in-addr.arpa as an MX record entry may be. regards, Ted Hardie