[dns-operations] Why would an MTA issue an ANY query instead of an MX query?

[Zuleger, Holger, Vodafone Germany]

> > "Not supporting"
> > ANY queries would also have side effects - simply dropping the
> > query maks the authoritative server appear unresponsive to the
> > recursive server initiating the query.
> 
> Note that in many cases the server receiving the ANY query is a
> recursive server, not an authoritative server.
What I have in mind is, that the recursive name server will send back
what is (actually) in the cache.
The clients doesn't know if the answer is full-fledged.
So sending ANY query to recursive servers is almost useless.