I also prefer plain text email. HTML email is no good, I think. However, you can use a multipart message with a plain part if wanted; Heirloom-mailx and other plain text email clients will then display the plain text part in that case.

You could specify "Content-type: text/markdown" if you want Markdown (although I don't know which email clients will understand that), although I think that plain text is OK.

I also dislike email newsletters or email discussion lists; either way, I would prefer NNTP.