Invalid base64 string for MIME content: The last straw
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.
Problem solved by deleting and re-adding the account. DOH!