Please consider adding Mailpile https://www.mailpile.is/ as a real alternative to Squirrel/Roundcube (which are horrid webmails).
Mailpile is a 100% free & open source modern, fast web-mail client with user-friendly encryption and privacy features.
Mailpile is still in a very early stage of development — the first "alpha" release, intended for developers only, just came out a few days ago. It is not ready for general use at this point.
We're keeping an eye on it as it develops, though.
It's been awhile now and Mailpile has continued to develop. Any updates on this?
It is still in beta, however, which means it's not ready for prime time.