There is an option not to display the HTML email messages... right?
Of course there is. Opening up a piece of mail in SimpleMail presents a series of icons beneath the text, just click on the TEXT icon for plain text, the HTML icon to read the HTML portion, and optionally the Datatype icons for viewing attachments.
While I fully understand your dislike of HTML mail, it's nice that we can finally read HTML specific e-mail comfortably from an Amiga mail client without having to fire up a browser first ;-)