Mailpile has existed as an app within Freedombone for a couple of years, and it's a nice webmail client, but for a more mass market type of approach it's not ideal. The reason is that the setup is quite non-intuitive and assumes that you know what acronyms like SMTP, IMAP and GPG mean. It's highly doubtful that the average shopper knows about any of that, and chances are they just use Gmail because that's what they were instructed to do by the initial setup process when they first got an Android phone. Gmail didn't ask them for an IMAP domain.
So I added a customized version of squirrelmail to the web interface of Freedombone.
The login has been changed to a new logo, and it's linked up to themes and languages such that if you change that on the settings screen the webmail system also changes accordingly. Testing it on mobile in the vertical orientation it looks odd but in horizontal orientation its ok and quite usable. I made a couple of themes called freedom_light and freedom_dark using the same colors as the main web interface so that it looks somewhat consistent. And you can use it to send between onion or clearnet email addresses without much hassle.
So despite its age and smartphone agnosticism Squirrelmail still appears to be quite a good addition.
Apart from the usual advantages of onion addresses the biggest one here is that you don't need to be using GPG to still have fairly good communications security. It's not end-to-end in the strictest sense, but a lot more secure than email usually is. You can also use it via a Tor browser with the security level cranked up to the max if you want to.