"John Woodard" mail@johnwoodard.co.uk
Agreed rejected mail or acid comment to list newbies will cause offence and not send out the right friendly message.
Please reject html email politely with directions to post as plain text only. Do not reject with an acid comment. If you want, you can forward their email back to them easily from the admin panel. If html email is a growing problem, you can make it clearer on the list signup page that html duplicates are not wanted, maybe with links to instructions for commonly-broken email software.
Several people on this list ignore or canned-reply to html email attachments (quite legitimately, in my opinion). That will not send out a friendly message either, if someone's first batch of replies include N off-list "don't do that again" replies.