> Once upon a time, back in 1996, there was a really unfortunate bug in the most popular DNS server software (BIND 4.9.3): it did not respond correctly to "CNAME" requests (that is to say, requests for any CNAME data about a particular domain name). This is critical information that an email server needs to know to do its job. Thankfully, there was a way to work around the problem: "ANY" requests. These requests ask the DNS server, essentially, for any and ALL information it has about the domain name in question, including CNAME information.
> These ANY queries have two big problems:
> As you might imagine, for big domains with lots of mirrors (e.g. gmail.com), that's a lot of information, and so the response can be quite big. Big responses pose two problems: first, it's a waste of bandwidth, and second, it can expose a bug in qmail's handling of large DNS responses (see the next patch).
> ANY queries are often not cached by relaying DNS proxies (for whatever reason), and so ANY queries cause more traffic EVEN behind a caching DNS proxy server.
Qmail, as opposed to other software, was sane enough to understand that lack of MX in ANY didn't mean it wasn't there. It meant it wasn't in a cache, so it would retry with just MX.
Our initial proposal (to do REFUSED for ANY) indeed had a chance to break some older qmail installations. This is why we engaged in a longer process and found acceptable solution - HINFO. HINFO is both backwards compatible (qmail will work fine) and solves our problems with ANY. Win-win.