Details Viewer is a simple macOS Mail plugin that shows more details for messages. For example, it can auto-expand the details button in message headers, for when useful information (such as Reply-To, for mailing lists) is hidden in the header. Details Viewer will automatically expand something like this:
Sound useful? You can download the latest release of Details Viewer from here. To install, simply unzip the downloaded file and copy the DetailsViewer.mailbundle folder to ~/Library/Mail/Bundles/. Restart Mail and Details Viewer should be loaded automatically.