So far, Protonmail has evaded us with eMail client support.
Now the option exists via the Protonmail Bridge, which is available for Mac OSX, Windows 10 and GNU/Linux.
Here’s how I set it up in Ubuntu:
From the protonmail.com/bridge/install page, click on the GNU/Linux to download the .deb file. Double-click on it and launch the software manger. Install.
Unfortunately, right now the only email client that works with this Protonmail Bridge is Thunderbird, which is a Mozilla product. I had previously deleted this app out of protest of Mozilla’s anti-speech behavior but since it’s free and open source, and the only option for now, I will have to live with it. If you do not have this program, in a terminal, type:
sudo apt install thunderbird
Before you setup Thunderbird, you will need to launch and configure the Protonmail Bridge. All of the instructions for configuration are here: https://protonmail.com/bridge/install
Then to setup Thunderbird, go here for instructions:
https://protonmail.com/bridge/thunderbird