VS Code Email Viewer allows you to preview EML and MSG files in a VS Code web view (virtual document) as well as mounts EML and MSG file as virtual workspace directories giving you the option of browsing the email attachments.
Click and EML or a MSG file and wait for the email preview to open. At the same time, a new workspace directory will be added - it is a virtual file system which allows you to browse the email content and attachments.
To reopen the preview after you've closed it, just reopen the EML/MSG again. To close the virtual file system with email content and attachments, remove it from the workspace.
See the changelog.
Perhaps the custom editor API (a read only editor) is a better fit than the web view API, but consider that this will block the user's option to show the actual email file content then, so for that reason maybe the extension should support both and make that configurable.
It only seems to happen the first time an email is opened, not the subsequent times.
Perhaps the webview needs to be allowed some security stuff for them to work or maybe webview messaging needs to be used and the file open request handled in the extension code.
The body text seems to be raw and only processed for display by Outlook (?) so we can try to render it as MarkDown to make it look at least somewhat nice.
If this is wrong and we get HTML it will just stay HTML after the MarkDown pass so this isn't risky.