How do I get rid of question marks enclosed by a square in my email?

It's a font problem. Launch Font Boot and validate all of the fonts that you have.

What specific fonts are used in those e-mails that show the question marks?

outtacontext wrote:

When I recently upgraded to Catalina (10.15.7) I started to get little question marks enclosed by a square in my emails (only emails I receive, not that I send).

While I have many fonts in Fontbook, I also use RightFont to manage my fonts. I used this with Mojave as well with no problem.

To trouble shoot further you can:

—Try a SafeBoot https://support.apple.com/en-us/HT201262

Takes noticeable longer to get to the login screen, does a 5-15 minute disk repair before it fully boots up, and certain system caches get cleared and rebuilt, including dynamic loader cache, etc. Login and test. Reboot as normal and test as caches get rebuilt.

In Safe mode third party system modifications and system accelerations are disabled, it removes malware, etc hampering smooth operation, however a reboot will put it back to normal mode.

This test will tell you if third party interference; extensions etc are not loaded in safe boot mode.

I don't think what got to mention which font IS used in those e-mails. That would certainly help clear things up.

I am thinking that they may have used a font that you don't have and maybe Mail failed to apply a successful font substitution.

Can you select some of those question marks and do Command-T to see what you get?

View->Show Fonts (or Command-T)