Hi johngar,
We see that you're having an issue getting the contents of your email loaded in some cases. Have you tried restarting the app and your Mac to see if the issue persists? This can sometimes resolve minor software issues. You'll also want to make sure that you're connected to a good Wi-Fi network to load large images and files. Does this persist if you check your mail in a web browser as well? This will help determine if it's the Mail app or your email server experiencing an issue. If things continue you can start your Mac in safe mode to see if there are any startup items impacting the issue. You can learn how to do that here:
How to use safe mode on your Mac - Apple Support
If your issue continues you can reach out for additional support here: Get Support
Take care.