If you experience problems with eM Client not behaving as default mail client for opening mail files, using Mailto links from web, etc. there might be a specific default setting missing in your system.
The best way to resolve it is to go to your System settings > Apps and then 'Set defaults by app' (Windows 10) or 'Default apps' (Windows 11).

From the list of installed apps select eM Client and then assign all available file and link types to make eM Client function fully as a default app for all email and calendar functions.

If you experience issues using the Send to > Mail recipient function in Windows, the attached registry file can help - simply download it and run it.