perhaps when the game has progressed a little further we can
solve the many-subdomains problem in one of the following
ways:
1) example.com TXT .... subdomains=_sub.%{d}
_sub.example.com TXT v=spf1 a -all
If a domain has no SPF record, move up the tree in search
of a record which has a subdomains modifier.
2) update the rfc to describe a default best-guess. in
other words, if a domain exists and has a or mx records
but no spf record, assume its spf record is a/24 mx/24
ptr.
3) change the dns rfcs to say that the implicit mx rule is
deprecated.
that means that an email address of the form a(_at_)b(_dot_)com must
require b.com to have an mx record to be valid.
Other solutions are possible.
A good solution is one that maps well to the problem.
(A good program is one that maps well to the solution.)
We should pick the solution that corresponds best to the
principle of least astonishment. I think #3 might work.