Installed Mailspring today and connected 8 of my email accounts to it, but will unfortunately have to uninstall. 
No way to sort inbox messages is a dealbreaker for me. Especially being unable to sort by sender, or by date, so now I’m seeing emails in reverse order. This appears to be a bug (or feature) that many people have asked about.
MainSpring is the only email client I’ve used that can’t sort the inbox, and it’s rather disappointing because everything else about the client (fast, simple, clean etc) is appealing.
I would gladly pay a monthly or annual subscription fee for this client, if it ever gets the ability to sort messages.
~James