Body of Messages not loading

Thanks for visiting the Apple Support Communities, DePen15,

To troubleshoot issues related to content loading in the macOS Mail app, we would recommend going through the following steps. They will help us isolate the issue, and work on common causes for such an issue. Please go through the steps one at a time, testing between each:

1. Quit the Mail app: Quit apps on Mac - Apple Support, then restart your Mac: Shut down or restart your Mac - Apple Support. Once it's back on, open the Mail app while holding the shift key on your keyboard. Try opening your messages after that to se if they load.
2. Make sure you are running the latest version of macOS: Update macOS on Mac - Apple Support
3. See if you are able to open the messages in safe mode: How to use safe mode on your Mac - Apple Support
4. Try to access your messages in a new user account: Set up users, guests, and groups on Mac - Apple Support

Don't hesitate to respond back if you still need help.

Have a great weekend!