Mac OS Monterey Mail - "Invalid base64 string for MIME content."

I have been a Mac user and Mac-certified tech since 1989 and NEVER have I been more frustrated with Mac Mail than with the Monterey edition! The issues of "Invalid base64 string for MIME content" and "blank" messages are driving me CRAZY!

I either get multiple warnings with the "base64" issue or a message appears in the Mail window but when I click on it, the body is empty!

These errors do NOT happen with Outlook! The problem with switching is that I have YEARS of reference data, custom spam rules, and folder hierarchy that would be lost if I had to switch to Outlook!

I have a MacBook Pro 14" running Mac OS 12.3.1 as the subsequent updates haven't been received well either!

Come on Apple get Monterey RIGHT!