I would like to tryout Pantheon Mail (a Geary fork by Elementary OS) on Ubuntu (15.10). I found their code hosting but I can't find how to install that. There's also a Q&A about building from source on Elementary, but those steps don't work for me (apt-get can't find the pantheon-mail package).
I've tried adding ppa:elementary-os/stable and ppa:elementary-os/daily but this doesn't help. The stable one gives a 404 when doing an apt-get update, and the daily one works but still pantheon-mail package can not be found.
Googling for [ppa pantheon mail], I only find explanations about installing the whole Elementary/Pantheon desktop, which is not what I'm after.
Is there a way to install Elementary's Pantheon Mail on Ubuntu? Is there a PPA? Do I need to build from source and then how to?