I have added the following section to my working copy of
the draft-ietf-mhtml-info document. It has not been
submitted yet.

1.1 Reducing the download time

If a message is sent as multipart/alternative, this would
normally mean that the mail client downloads both variants,
and then shows only one of the to the user. This will thus
increase the download time. A way of avoiding this problem
is to use the FETCH command of IMAP, which allows a client
to download only certain body parts from a multipart
message.
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Jacob Palme <[log in to unmask]> (Stockholm University and KTH)
for more info see URL: http://www.dsv.su.se/~jpalme