Re: message/external-body mail-server access type

Well, the message that comes back will itself presumably be in MIME
format, so it shouldn't be a problem to extract the data or tell where
it is.  But you're right that there's no way specified for associating
it with the external reference.  Perhaps we should add such a facility. 
One very simple mechanism might be to use the Content-ID field -- that
is, to say that the thing that comes from the mailserver will have the
same Content-ID as the external reference to it.  Does that sound
reasonable?