I have not yet discovered the cure for MacMail opening messages and not displaying their content. At this point the only workaround has been to quit and restart Mail between each message. I have rebooted Mail with "shift" depressed, I have rebooted the entire system in safe mode, I have deselected privacy options in Mail preferences, Network System Preferences that Limit IP Address Tracking is unchecked.
iMac 21.5″, macOS 12.6
I and others have the same issue with Monterey and Ventura. I'm afraid there is no solution at this point. Apple needs to fix. I now use Outlook which has no issues. Review this discussion:
macOS Ventura Apple Mail messages are bla… - Apple Community
I already tried that and no impact. The only choice is to hope Apple fixes the issue or use another mail app like MS Outlook. You should give feedback to Apple. The more that report the better chance to get it fixed:
It seems the problem has been gradually getting worse, like an accumulation of junk, since I updated to Monterey 12.6.3 on Jan. 10, 2023. I thought about re-installing the system, perhaps that could help?
Mail messages not displaying content
