SPF macros do indeed provide the functionality I'm looking for. I'm
just trying to justify why I think this functionality should be
considered for inclusion in MARID.
SPF macros are on the verge of being Turing complete.
I think that it is a pretty good thing to ask what the use cases are and
whether they justify that level of complexity. SPF design was constrained in
ways that do not apply to MARID. SPF had to pander to people asking for
features to avoid splits. I think MARID has the opposite issue, the bias
here will be for simplicity.
Over complexity and over simplicity are both equally bad. The main reason a
lot of specs become complex is when an inadequate design is extended
incrementally (see MTP for an example).
One issue that does concern me is the extent to which it is practical for
enterprises to maintain separate per email address data through the DNS. At
the very least I think we need to describe a mechanism that allows the
management of per user information to be decoupled from general DNS
administration (i.e. on a separate server via a zone delegation if the admin
chooses).
Before we get to the how though we have to decide whether. If this is going
to be practical and usable it has to be widely supported.