Generating Text-only version of Newsletter

Hey there, I've been trying to optimize my emails to avoid SPAM folders and one of the dings I'm getting right now (per mail-tester.com) is that I'm only sending out an html version of the email and no text version (more specifically, the error reads "Message only has text/html MIME parts"). I did find the page under "Edit Newsletter" where you can edit the textual version of the newsletter, and there's a warning bar above that that says "The text part is sent only when Newsletter manages directly the sending process." with a link to an "Advanced Settings" panel that doesn't seem hugely relevant at first glance.
Anyway, as far as I can tell Newsletter does directly manage the sending process for my messages, but there's no text version included (which I verified on a test email with gmail's 'show original' button). Any thoughts on how I can force the textual version to be sent?