In case you've not been following it, here is a link to this month's blogpost on the Mailpile projoect.
As you'll see, it seems that they have solved their "font problem" by forking a truly FOSS font, and making various tweaks. The resulting font, of course, will be free for other people to use and adapt in their own FOSS projects.
They are now also working on OpenPGP integration - one of the promised features that appealed so much to their IndieGoGo backers.
Interestingly, the people running the Cloud Fleet project https://cloudfleet.io/ , a kind of future freedombox type system, are now quoting Mailpile as the bundled maill app, alongside owncloud and etherpad.
David