I believe Riseup uses Roundcube as webmail. Last time I checked, both the
backend PHP and the client JavaScript code of Roundcube are free software. If
Riseup does any bug fixing or feature implementation, I hope they're doing it
upstream or they publish their modified version of Roundcube.
I've had a quick look at ProtonMail and it seems to have free JavaScript
client code. I believe we can get a copy of that Expat-licensed code by
running:
git clone https://github.com/ProtonMail/WebClient.git
I'm not sure if they've released the backend software. Probably not.
(Also, ProtonMail seems to want to provide IMAP/SMTP for desktop clients, but
haven't managed yet:
https://protonmail.com/support/knowledge-base/imap-smtp-and-pop3-setup/)
I haven't checked OpenMailBox yet.
