MacMail font issue

Do the following as needed:

1 - boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

2 - launch Font Book, select all Fonts, Control (right) - click on one of the selected fonts and select ValidateFonts from the contextual menu:

Please go to Mail Preferences -> Fonts & Colors and take a screenshot and post so we can see what fonts you have configured there.

Here's the default settings:

The only thing else I can think of is perhaps fonts being setup for international?

In the Font Validation window select "Warnings and Errors"s from the first drop down menu.

That will give you those fonts that have issues.

I've set all of my fonts to this:

I had it set to Times New Roman before without any problem. I've set the size up there due to eye problems.