Hello,
Welcome to our Microsoft Q&A platform!
Yes, this is the expected behavior.
Windows.ApplicationModel.Email is not supported in desktop apps and is not on the list of API supported in a packaged desktop app.
For more information, please check the following article:
UWP APIs available to desktop apps:
https://zcusa.951200.xyz/en-us/windows/apps/desktop/modernize/desktop-to-uwp-supported-api
In order to implement your requirement, you may need to include both a UWP and a desktop app in your package and call the email API from the UWP side. Please check the following articles to the get start:
App Service:
https://zcusa.951200.xyz/en-us/windows/uwp/launch-resume/how-to-create-and-consume-an-app-service
App Service Destop Bridge sample:
https://github.com/Microsoft/DesktopBridgeToUWP-Samples/tree/master/Samples/AppServiceBridgeSample.
Thanks.