Goodwill alone wont work. If you want all mail to be plaintext, the list admin needs to set the list server to reject html mail.
I switch between html and plain ascii on my GMail account several times per day -because some stuff needs to be with formatting- (I use the web -AJAX- interface, btw), and sometimes forget to switch it back to plaintext.
When I used Mozilla SeaMonkey, it was all easier, because it allowed you to specify which lists (or addresses) to send mail as text-only and which ones to send as html.
But hey, we are on the Cloud now. So the user has fewer and fewer control about his software, its all decided by the know-it-all experts in the Google ivory tower.
FC -- "Si, yo creo en los objetos. Inclusive más que en los seres humanos. Creo que son más fieles ¿no?. Porque, claro, los seres humanos pueden traicionarte, pero los objetos no, a los objetos los traicionamos nosotros." -Manuel Mujica Laines