I'm also exploring Buttondown, a newsletter mailing list software, which uses Markdown as its native format for composing emails, as part of my job managing a WordPress site for a non-profit. Buttondown can subscribe to any RSS feed and turn the items into a draft email (with the items' content converted from HTML to Markdown) that you can then tweak before sending it out to subscribers. I was able to create a custom RSS feed out of WordPress that has source:markdown content for its items, and now I'm trying to convince the Buttondown people to parse that out and have it appear in the draft without modification.