Invalid base64 string for MIME content - the final straw?

I’d really appreciate any insight or help here chaps.

Apple Mail is now throwing up the same error after error after error, rendering it useless. It is failing to move messages: “Invalid base64 string for MIME content.” The only way out is to Force Quit. As a Mac and mail user of some 30 years I am forced to give up on Apple Mail.

This is just another, albeit more serious problem, in a long litany of bugs and irritations with Apple Mail over the last year or so. These include getting stuck downloading simple emails; inconsistent rule behaviour and frequent crashes.

I’m frequently deleting and re-adding accounts; re-installing through OS updates; cleaning the message cache and re-building mailboxes.

Assuming macOS, yes? (Important, that.)

Try removing your mail rules, (copy/screenshot to save, if desired), quit, then relaunch.

Problem solved by deleting and re-adding the account. DOH!