Brucehankins I tried and tried to like Mailspring, especially as they went opensource. I know Vivaldi is not fully opensource. That being said, it just acts up with the protonmail bridge too much for me to choose Mailspring full time. I hate having to hit F5 every time I look at my proton account and mail disappearing and re-appearing at random times. Thunderbird has a nice unified mailbox although I am completely torn on the whole Firefox, Mozilla, Mitchell Baker thing...ok, sorry, off topic.
@brent Thanks for that Forbes link!