Email Norification: Invalid base64 string for MIME content

This is not a solution, maybe a work around. Moving you whole user account over to another account cannot be apple’s fix. Most users will not know all that is needed to make the move. Apple doesn’t want to admit it may be a Catalina BUG.

Having the same problem. I have two Exchange accounts but this is only happening with one of them. Spent over an hour with Apple technical support trying to figure this out. Rebuild, delete account, etc. multiple times. Nothing worked. PLEASE FIX THIS!

I think I was one of the first to identify this issue with Apple. At least they said that I was the only one having an issue. I was missing important emails and sending emails which were not being received or going to Junk. I tried to resolve, but was forced to move to Outlook. I haven't worried about this issue since.

I just want to caution people, with this issue, that you may be having more problems than just the annoyance of having to acknowledge a problem moving an email.

emmotive wrote:

I found a solution for mine (which has so far resolved the problem) - turned out to be simple, but I suspect it was a bit brutal on the system.

I went to the Library folder using the Finder 'Go to Folder':

~/Library/

Then simply deleted the whole 'Mail' folder

Did this, but it now happens again to me.

So this did NOT solve the issue for me.

This is not a fix but a work around. Only Apple can supply a fix. Apple has already put out three newer versions of Catalina without a fix to this problem. This work around has been known now for about six weeks. This is what happens when folks don’t read the whole thread.

Computer was fine — and has been (shockingly) relatively stable under CATALINA 10.15.3 for months.

Woke up this morning with the AppleMail First Login Screen (Would you like me to import messages? — or whatever it says). That without turning computer off or any substantive modifications to configuration or workflow. Everything went pear shaped from there. It seems SOMETHING was horribly corrupted.

Logic said take the config back to a pristine, virginal state

Same issue for mails send from a mailbox connected to an Exchange server (on prem, not Office 365)

Same issue here. Apple needs to get it together. I am near converting back to PC.

same issue for me as well

I'm having the same issue!

This is not a fix, if it doesn't work for everyone it's not fixed.

Did this work for anyone else?

This solution worked. Thank you!!! This problem has been such a hassle for months (public beta onward).

Nevermind -- the problem returned. After only one (glorious) hour of being problem-free. Apple please fix this ASAP.

I have the same problem...Only since Catalina was installed.