For proper setting of Mailspring as a the default email client for Windows, the email client is required to support Simple-MAPI.
As the current version does not, so emailing does NOT work from Windows File Explorer (ie. Send to…email recipient), and from all other Windows programs that provide a “Send as email attachment”.
This applies to all Windows versions (7, 8.x, 10)
This may seem like an old technology (which it is), but many new printers that have a scan-to-email function in their software tools, requires a MAPI complaint default mail client.
(Originally posted by michel-cote on GitHub.)