To set eM Client as the default Mail client using Windows 10, see the following thread.
https://forum.emclient.com/t/emclient-reports-its-not-default-email-client/49613
To set eM Client as the default Mail client using Windows 11, see the following thread.
https://forum.emclient.com/t/windows-11-making-em-client-default-email-client/78206
Note: Reboot after doing either of the above.