I had the same problem as others when I bought a new computer with Windows 10 in that eM Client stopped working and I couldn’t set it to default. It is now working fine since I downloaded net 2/3 and changed the port settings. These are now SMTP port 587, select ‘Force usage of SSL/TLS’ and IMAP port 993, select ‘Use SSL/TLS on special port (legacy)’. Although it now appears in Windows as the default application I still can’t set it as this from inside eM Client but this doesn’t seem to matter. Hope this will help someone else!
Note in windows 10 EmClient cannot set itself to default, one needs to got to Windows Settings - System - Display_Apps then you can set “EMAIL” to EmClient .