I signed up today. It's pretty slick.

http://imgur.com/a/Xt4CQ

Looks like they are using Rainloop (https://www.rainloop.net/) with a customized skin.

I'm a bit suprised: While Rainloop seems to the most modern opensource webmail client I could find (and I did do some research), from their github: _This is NOT a stable version of RainLoop Webmail. It's not recommended to use in production environment._ From my own testing there are certainly still a few bugs. Interesting they go with this for their primary webmail.

If they are using the AGPL community version of Rainloop, that might also pull in some obligations they aren't meeting.

AGPL is pretty viral, even for a web app.