We can distinguish general "computer programs" from those
that specifically handle RFC [2]822 messages. While it certainly
seem to be true that it's desirable for the URI conveyed to be
usable to *some* computer programs (viz. URI handlers, a.k.a.
browsers), it is not necessary for it to be part of some structure
which is handled by RFC [2]822 message-handling programs.
I.e. the URI can be conveyed in an unstructured field as far as
RFCs [2]822 are concerned, while still being a URI.
yes it can. but as soon as we might want MUAs to be able to use those
URIs for specific purposes (like accessing archives from within the
mail client) and to associate other information about those URIs (like
a description of the format of the archive) then it becomes useful to
have archived-at be a structured field.
Keith