Invalid base64 string for MIME content - the final straw?

Tech Support
1

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.

2

Assuming macOS, yes? (Important, that.)

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

3

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