Invalid base64 string for MIME content

Has anyone found a fix for this issue? This has been unsolved in the Community since October 2019. Did anyone get a fix? I am having this problem with some of the Exchange accounts that I check in Mail. This only started on my MBP (late 2015) after I combined some Exchange accounts under one account in Mail to eliminate the clutter.

There has been no action on this question. So here I am starting a new string. Any help?

Catalina was the closest choice. Mail was not an option.