Identify which rule causes this error in Apple Mail: "Invalid base64 string for MIME content" and how to fix?

To Apple Mail I added 2 Microsoft Exchange accounts (one for business; one for my nonprofit) and one Gmail account (personal). Correspondingly, I have 3 different email addresses. Often, people use the "wrong" email address to send me a message, and often I receive multiple email messages from the same address, and want some subset of them. This is why I created several Rules in Mail.

The idea is that the Rules will route or reroute every message to the correct inbox, to the "Archive", "Trash", etc. It's a big help when I'm trying to shift gears between projects. To move an email message using Rules, sometimes a particular word must be in the subject, sometimes the msg must be from a particular email address and sometimes it must meet some combination of criteria.

Rules mostly work, except when they don't (like this morning). Then up pops the "Invalid base64 string for MIME content" error for every email message Mail was asked to reroute, even those that were previously sorted! When that happens, I have no choice but to force-quit Mail. One time I hit OK for every message it was complaining about and it took about 60 clicks before it stopped. That's about the number of msgs in all folders that were rerouted at one time...

Did I just figure out the problem??? Many messages that Rules rerouted to the correct inbox, get sorted further into one of many folders, "trash", or even "sent". Could the app send the error message because it moved an email message according to the rule, and then I moved it somewhere else?..but how do I possibly fix that??

Is there a way to make Rules apply to new messages only?

THANK YOU for any help you can provide!