More BOFH-ish?..
'E-mail demons' always use HTML mail because they can then show off their l33t text formatting skillz (and use them to cover-up their less stunning writing skill).
Sooo.. I used to just bounce such mail back to the sender (and the CC list) attached to a plain-text e-mail containing a boilerplate response..
"This message was not delivered because the recipient's e-mail system does not accept HTML formatted e-mail.
HTML formatted e-mail is generally spam, malware or other unwanted e-mail in which the formating of the content is more important to the sender than meaning, security or efficiency.
If this message was actually important or business related please re-send it as a plain-text e-mail, rewritten in a manner that does not rely on HTML formatting to convey it's meaning."
.. I'm particularly proud of the phrase 'important or business related' although now that I think about it the intended meaning of that would have been clearer if the word 'or' was in bold..