That was embarrassing!
The first muse-letter was sent out earlier but went as raw Markdown rather than nice HTML. Oops.
I'll stick my hands up, I forgot to add Markdown support to the custom post type but that's now fixed ready for next time.
As is the way with WordPress, everything is available online (the first letter is here) and it automatically creates an RSS feed of the post type but the muse-letter is designed to be viewed, and interacted with, by email.