I'd be tempted to make all external references be of the form
[ref: some-content-id], where the content-id must refer to a
content that appears in the same message as the reference. The
content-id could, however, be that of a message/external-body content...
that way any extensions to the MIME external reference mechanism are
automatically incorporated into simplemail. Mail readers would have
to provide hooks for this, but I'm sure other content-types will want
to be able to do this kind of referencing also.
I agree with Keith on this; I think it builds nicely on the MIME multipart
facilities that are already available. Note that you can also reference a
Content-Id: on a multipart/alternative if multiple versions of the same
[sub]document are available.
I also think the ability to include a single document in multiple places will
be more useful that you might think at first. The idea of providing multiple
ways to navigate through information is a very powerful one.
Ned