A few more data points:
* Is multipart/alternative widely implemented for generation?
The Andrew Messages program generates multipart/alternative routinely
with no explicit user action required.
* Is multipart/alternative widely implemented for reception?
Again, Andrew Messages does this right.
First Virtual has also built a gateway that converts
multipart/alternative and other stuff to non-MIME text mail, which might
be considered an implementation of the reception of
multipart/alternative.
* Are Nested Body Parts widely implemented?
Andrew Messages handles these well upon reception, but does not generate
them. Safe-Tcl programs generate them routinely, but Safe-Tcl does not
provide general-purpose composition tools at the user (as opposed to
programmer) level.
* Are External Body Parts widely implemented?
Metamail handles their reception very well, and its companion "mailto"
program is easily configured to facilitate their composition. The
metamail program also includes a simple special-purpose program for
composing external body part mail.
--------
Nathaniel Borenstein <nsb+faq(_at_)fv(_dot_)com> D3789F3368C124D5
Chief Scientist, First Virtual Holdings 7AEDD0DC859E8C02