I go to Menu > Tools > Settings > General and scroll to the bottom where it says “Default Email Application - This application is not the default email handler.” so I click the “Make default” button and nothing happens except the screen darkens and Windows 10 puts up a notice “To change your default apps go to Settings > System > Default apps” and there is an “OK” button to remove the notice. So I do as it says and make “eM Client” the default app for email, but it does not change the indication in “eM Client” itself to show that it is now the default email app., so presumably it is still not the default. Any ideas on how to solve the problem? 
Hello Jeremy,
this seems to be a common Windows 10 issue where it does not keep its own default setting.
Go to Control Panel\All Control Panel Items\Default Programs\Set Default Programs and click on eM Client.
Click Choose defaults for this program and select and save all the options.
eM Client should have all the defaults then and work as default properly.
Regards,
Olivia