Not asked, but equally important: Can you describe how to observe this issue?
Some answers:
- Use a mail user agent (MUA) that prefers text/plain parts to text/html parts. (I use a heavily customized version of
mailx, your milage may vary.) - View the mail file on disk in a text editor to see how it is constructed.
- Use power tools in your MUA of choice to look at other MIME alternative formats.