The best mail client on Linux. Beautiful. Modern. Minimalist. Well designed. You can really rely on it. It's really useful. There have been serious problems with the flatpak before, but since version 3.36.2, the flatpak works flawlessly.
I quite like the simple design of this app, but like @nowakowski mentioned, the application is seriously having some issue, with version 3.34.2 running on linux mint 19 cinnamon. So far I only got it run once, that's when it got installed. After that it just gave me a blank screen and the application just got frozen. [Update] I found a workaround. I went into the "System Monitor" and kill Geary process and re-launch the app and it works. This has to be done everytime I restart the computer. Found another issue after this, I notice that Geary does not support Chinese input.
I really like the application except this bug the reviews before me mentioned. I found one thing useful, until this bug is fixed: If you run the application from the console first with flatpak, it works perfectly fine.