The site was created with a new Sitefinity CMS 15.4 project and a decoupled ASP.NET Core renderer. This is not an upgraded project; only content/data has been migrated.
When creating an Email Campaign Message Template using “Like a web page”, the Content toolbox is completely empty. The editor displays:
“Drag form widgets to add fields to the form”

In Administration → Settings → Advanced → Newsletters, Enable Web Forms framework is disabled. This is intentional, since Web Forms is deprecated.
Under Administration → Settings → Advanced → Toolboxes → NewsletterControls → Sections → Common → Tools, there are standard controls registered, including:
ContentBlock
ImageControl
NewsView
BlogPostsView
ListItemsView
EventsView
MediaPlayerControl< br>VideosView
DocumentLinkControl
DownloadListView
GenericContentView
However, these are Web Forms controls, not an MVC widgets. 

What is the officially supported way to enable/install the standard MVC widgets for Email Campaign Message Templates in a new Sitefinity 15.4 Headless + ASP.NET Core Renderer project?
Is an additional package, feature, module, or project configuration required?
Can the built-in MVC News widget be used in an email campaign template to select and render existing News items without enabling Web Forms?

There are no browser console errors, failed network requests, or server-side errors.

The behavior is by design for lightweight headless installations.

The 'Progress.Sitefinity.Headless' package intentionally excludes assemblies and modules that are not required for a lean headless deployment, including MVC-based widget infrastructure used by the Newsletter module. As a result, the 'Like a web page' template editor has no MVC widgets to load into 'Toolboxes → NewsletterControls'.

Commonly missing infrastructure includes:
- 'Telerik.Sitefinity.Frontend'
- 'Telerik.Sitefinity.Feather'
- Legacy MVC-based controls used by Email Campaigns / Newsletters

1.Reinstall the CMS using the 'Telerik.Sitefinity.All package and run the project in hybrid mode.

OR

2. Install Telerik.Sitefinity.Feather on an existing Headless CMS project (e.g. Install-Package Telerik.Sitefinity.Feather -Version 15.4.xxxx -IncludePrerelease).
Build the solution and make sure there is no error. Save the project.

   2.1 Do IISRESET /STOP

   2.2 Remove all the subfolders for Temporary ASP.NET Files folder under C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root\
   
   See Notes section for more information.

   2.3. Do IISRESET /START 

Navigate to the .NET Framework directory and delete only the contents (subfolders and files) inside the Temporary ASP.NET Files folder. Do not delete the root folder itself.
The paths vary depending on your application framework version and system architecture:
- 64-bit .NET 4.0 / 4.5+:
C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files
- 32-bit .NET 4.0 / 4.5+:
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Temporary ASP.NET Files
- 64-bit .NET 2.0:
C:\Windows\Microsoft.NET\Framework64\v2.0.50727\Temporary ASP.NET Files
- 32-bit .NET 2.0:
C:\Windows\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files