I have tested Netscape 4.04 and Eudora 4.0.1 to see how
they use Multipart/alternative for a message with a HTML
document with an
embedded graphic. Result:

Netscape put Multipart/alternative outside Multipart/related,
while Eudora put Multipart/related outside Multipart/alternative.

I think the choice made by Eudora is slightly better, since it
might mean that some recipient mailers, which can handle MIME
and attachments, but not Multipart/related, will still show the
image as an attachment to the plain text version of the message.

How do other mailers handle this?

It is obvious, since two major mailers do it in two different
ways, that mailers on receipt must be capable of handled both
ways.

I enclose my test messages as an attachment to this e-mail.
The attachment also includes a test with Microsoft Outlook
Express, but that test is not comparable because I did not
know how to put images into messages sent with that mailer.