How do I make Mimestream the default email client?

Unfortunately, Mimestream cannot offer an option to become the default email client within the app. Mimestream is a “sandboxed” application to provide users with enhanced security, and macOS doesn’t allow sandboxed applications to use the APIs necessary to change the default email client.

Workaround: Using Apple Mail

Open Apple Mail to change the default email client, following the instructions on Apple’s support site.

Note: if you’ve never used Mail before, you will first have to add an account before you can access the app’s preferences.