In Thunderbird 3, when I receive a message with a smiley using the "Original HTML" or "Simplified HTML" viewing option, it appears like this:

That is … J. However, if I view the message as "Plain text" I get the smiley

The actual message in the "view source" looks like this:
Content-Type: multipart/alternative;
boundary="_000_7EE28"
MIME-Version: 1.0
--_000_7EE28
Content-Type: text/plain; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable
Se r=F3n... :)
That =F3 is an "ó" (I've censored the message for the easily offended Spanish readers out there). Maybe it is something to do with the encoding? I'm on Linux and use LANG=en_US.UTF-8.
Anyone know what has gone wrong here and how to fix it?
EDIT: I've tried switching encoding to ISO-8859-1 and forcing this, but no joy.

After further investigation, it seems that the Wingdings thing is the issue. The actual HTML has the following snippet where the smiley should be:
<span style='font-= size:11.0pt; font-family:Wingdings'>J</span>
... since this is a multipart text/HTML message and I had only added the plaintext part above.

