I believe Pine and IMAP4 servers (I know of at least 4 independant
implementations) are examples of handling nested multiparts on the client
side. Pine numbers the parts in such a way that the nested structure is
clearly visible to the user. IMAP4 parses the MIME message into a
hierarchical structure which preserves the nested represntation.
PMDF MAIL does this as well. So do the various DEC MailWorks clients, and while
they don't support MIME directly they support it just fine indirectly through
our MIME-->Message Router gateway. Message Router is one of the few non-MIME
environments around that actually handles nested structures reasonably and
completely, even when you nest them very deeply. (Please don't try to tell me
that X.400 agents generally support this too, as I have way to many experiences
with X.400 MTAs that fall over dead when presented with nested messages to
believe it.)
There's also ALL-IN-1, which again doesn't support MIME directly but does
support it indirectly through both our gateway as well as those of several
other vendors. And ALL-IN-1 has to be the heaviest user of part nesting
anywhere -- it routinely generates three levels just for what it regards as a
simple attachment, and this quickly multiplies when you reply or worse, forward
and add a cover note. I've seen casual ALL-IN-1 users produce MIME structures
20 or more levels deep with no effort at all, and heaven help the non-ALL-IN-1
user who has to deal with such messes. (Support is one thing, presenting suc
complex objects so that casual users can deal with them is quite another. I've
also seen these structure blow some agents out of the water, but plenty of
others handle such things very nicely.)
Ned