This is one way to convert an HTML-only email to a text-only email.
It is a last resort for when no safer and easier solution can be
found.

WARNING: Converting HTML to text is error prone.  You may lose the
content of the email.  If the email is at all important COPY it to
another mail folder and then try the conversion on that copy.

NOTE: These instructions assume you have not previously used any
mail filters.  If you have already used mail filters it's up to
you to figure out how to create a new mail filter instead of
following these instructions.  These instructions convert the
default "<unknown>" no-op mail filter to an UnHTML filter.

1. Settings / Configure Filters.

2. At left rename <unknown> to UnHTML.

3. In General tab select filter action "Pipe Through".

4. Copy/paste the following in the white box all on one line.

perl -e '$h=1;while(<>){print;if($h&&$_ eq "\n"){$h=0;open(STDOUT,"|
lynx -stdin -dump")}}close(STDOUT);'

5. In Advanced tab uncheck all, except check only "Add this filter
to the Apply Filter menu" and "Additionally add this filter to the
toolbar".

6. OK

You should now have a blue gear wheel in your toolbar and the
tooltip for it should say "Filter UnHTML".

Select the message you want to convert (or for safety a copy of
it) and click the blue gear wheel.  The message should disappear
and then reappear without HTML a few seconds later.

WARNING: If you lose information it's your fault for not making
enough backups.

Good luck!

--Mike