Hello Welles,
It seems we're not quite on the same understanding as to what's happening, as you mentioned now that text is not rendering and images do not have any issue. You also reported in your first message that the opposite is happening. What exactly is happening at this time, and for what content exactly are the question marks showing in your emails? The more details you provide, the better equipped we'll be to assist.
If you use another device, or even accessing your emails from their respective email provider websites, what occurs?
While the latest for compatible models is macOS 12.3.1, the latest for your current version is macOS 10.15.7. While it's not necessary to update to macOS 12.3.1, we do recommend making sure macOS 10.15.7 is currently installed. This link will show how to first back up, then Update macOS on Mac.
Cheers!