Why would sending mail in Wanderlust be successful in XP, but fail in Linux when using a Sun server?

[Endless Story]

I have already queried the Wanderlust mailing list on this question,
but no one has been able to help.

I'll try to put the problem simply - I can add detail later if anyone
has any thoughts.

I dual-boot (probably for masochistic reasons), and on both XP and
Linux I am running Wanderlust in Emacs 22. I am fairly confident the
problem doesn't relate to any bug in Emacs 22 - on both platforms it
has been perfectly stable for me. Besides, the  problem occurs only on
the Linux side (Gentoo), and I can reproduce it in Emacs 21 as well as
in 22.

Essentially, when in Linux, if I try to use Emacs/Wanderlust to send
mail to my ISP's server for SMTP (which happens to be a Sun), I can
login in and authenticate OK, but when it comes time to send the data,
the operation times out - the server is waiting for the client to
start sending, but it never happens. No problem on the XP side with
this same server.

Weirdly, the problem only involves forwarding or replying (with cited/
quoted text) to a non-plain-text message! I have no problem at all
with plain text messages - I can forward these or reply to them
without a hitch. And I can send an original message with MIME types -
for example, a PDF attachment - without a problem. Very weird.

The reason I think the server is important is twofold: First, I can
forward/reply using the SMTP server for a different ISP, whose server
is NOT a Sun, and everything works fine. And second, inside some of
the SMTP sending code for Wanderlust there is a little comment saying
that the next couple of lines of code are to test whether it's a Sun
server that eats newlines - in which case an extra newline must be
added between the head and body of the message. Not sure why this
would be a problem only with non-plain text forwarding/replying,
though.

I have Googled for hours trying to find a potential cure - changing
the buffer to DOS to get a DOS line ending, setting the language
environment, setting the coding system for network interaction to
various values, etc. Nothing makes a difference.

The one thing I haven't tried yet is to set up a different mailer in
Emacs, such as VM or even RMAIL, and see what happens when I try to
forward/reply under those conditions, using (I assume) the basic SMTP
module for Emacs.