Invalid base64 string for MIME content.

I was on the phone for 2 1/2 hours this past Sunday with Apple support about the base64 MIME error code. Apple admits it’s a problem in the IOS. Currently there is no fix for this issue. There is a workaround; you can set up another user ID on your Mac

and set up your email in that user ID. It does work fine which shows that the problem is not in the mail APP or in my case Exchange. Not a viable solution for me. This bug has been sent to their highest level of support. We’ll see what happens.

I too had the same frustration once I upgraded to Catalina. I was even given the same solution (as CatalinaProblemswithAppleMail) by Apple support to create a new user to see if that fixed the issue...it did but it was not practical for me to use the NEW user account just so my emails worked.

So after several hours on chat with Apple, I ended up finding the solution myself through another apple thread where someone else tried the following: (I told the support person to document this on their end so when others contact support they can at least offer one viable solution.)

Go to your email preferences - go to RULES - DELETE all your rules - issue FIXED...I haven't added any email rules back to see if it still glitches as I am just happy to have it working ...for now!

I had the same problem in the macOS Catalina Mail app with an Exchange account only. In my case removing a whitelist email rule, which moved emails from known senders into my general Inbox, is a workaround for this bug.

I was getting the same thing and got tired of clicking OK 100's of times per day. For me, this worked... In Mac Mail, I went into preferences, then rules. I deleted my old rules, then re-entered those same rules. It has been working for 24 hours.

I deleted the Rules in that account and it has been working properly for 2 weeks now!

I had the same issue and fixed it by disabling a rule I had that directed all email to the inbox. Since I disabled it I have not had an issue. Maybe try dialing all the rules and slowly add them back.

I had to take my exchange emails off. I completely deleted all emails, plist, preferences and it still came back when I added specific exchange emails.

I have six emails. I added one back at a time. Noticed that it was two of the six wreaking havoc.

Then I found this article by this guy.

https://buymymonkey.wordpress.com/2019/10/27/osx-catalina-cpu-spike-checking-mail-after-upgrade/

Please, don't do any of this unless you're running out of hope.

What has worked for me after adding one email back is/was #11

This right here

11. Uncheck “Automatically manage connection settings”.

However, with two of my other exchange emails it went nuts again. I believe it's one of two things. It's either looking for folders and emails it once had or it's looking for something else.

This guy on that article had a good point with Keychain, but it didn't help me. I didn't see any ghost key.

Tomorrow I'll add back two more of my six. See how it goes.

I believe this issue is related

work around is to delete/disable your rules. It works!

I unchecked the boxes under rules and it is working fine now... F I N A L L Y!!!

Worked fir me. I was able to do new rules and it is still working. Not sure if a Specific rule was the problem or what. But deleting them did fix the problem.

Same problem here, but only with emails received through my university Exchange account. Contacted Apple about 4 days ago and had a shared screen session. No solution. They contacted me back again today. Apparently, their "engineers are working at a solution" and they expect it to be in an upcoming update. So I guess we have sit back and wait.

It's exactly what I did, but I also turned off Under; Mail>Preferences>General, "Check of New Messages" and changed to manually instead of "Automatically". This seemed to keep accountsd from going into 400% CPU whilst stopping everything else on the system.. this is sooo stupid and unbelievable that it has still not been addressed

Make sure you call apple care

Using the Mail APP go to Preferences then Rules and uncheck any Active rules. Quit the mail APP, then restart it.

I only disabled the rules, unchecked the box for each rule. I have not tried to recreate the rules.