Email Norification: Invalid base64 string for MIME content

OK. I called Apple Support. Here's what we did and so far the issue is gone.

1. Reboot into Safe Mode
2. Open Mail and send myself an email from a personal account to the Exchange Account to see if the error came up.
3. No error in Safe Mode
4. Open System Preference/Users and Groups
5. Select "Login Items"
6. Delete all items marked "Unknown" In my case there were a couple of old items from software I deleted long ago.
7. Reboot into Normal mode and test again.
8. No more error.

Lets see if that holds. Good luck!

Something that helped me was removing my Rules. I had some rules setup not to miss emails from certain clients that were going to my junk mail or straight to deleted. Once I removed all of my rules, I resolved the problem. This completely solved my problem with Exchange and Mail (Catalina 10.15.2).

HEY EVERYONE! Apple released an update yesterday to Catalina 10.15.1. The problem is gone after the update. Give it a go.

So sorry to hear that - how frustrating. Mine is so far still working. But I did also have success with the solution suggested by ‘CatalinaProblemswithAppleMail’ (creating a new user account on MacOS but adding exactly the same Exchange account to it). That was part of my diagnosing that it was probably user-related rather than systemic mismatch between MacOS and Exchange (and so to me deleting the whole folder... just couldn’t face transferring everything to a new user account if just deleting the Mail folder worked... we’ll see.

To summarise, both deleting the Mail library folder AND adding a new MacOS account with the same email details worked so far for me. It’s frustrating to read that the solution I settled on may be temporary. I too hope Apple engage to resolve this.

I removed the Exchange account and re-added it, and that fixed it for me.

I removed the Exchange account and re-added it; that fixed it for me.

THAT WORKED!!!! I removed some rules and restarted mail and that solved my problem with Exchange and Mail.

Thanks

OK. If the upgrade didn't do it for you, I did follow the steps in this article from MacPaw:

https://macpaw.com/how-to/fix-macos-catalina-problems?utm_source=macpaw_digest&utm_medium=email&utm_content=button&utm_campaign=macpaw_digest_october19&utm_term=cmm3_active

Specifically, I deleted the 32 bit apps that no longer work with the new version as Apple now requires 64 bit apps. I also feel that it helped to have the software search for leftovers from programs I had deleted some time ago. It searches for updates to programs you have and it does a good job of deleting programs I no longer use. I followed pretty much all of what it recommended. Rebooted and I o longer have the problem. I did NOT do the PRAM/SMC Reset so that isn't what fixed it. Although it wouldn't hurt. In addition to stopping the problem, using CleanMyMac X seems to have smoothed out quite a few wrinkles and my Mid 2012 MB Air seems to be running a lot better.

Post back if you go through this process and it works for you.

Did some experimenting.

The fix:

1. Force quit Apple Mail (if it's constantly pushing that popup)
2. Open PREFERENCES->INTERNET ACCOUNTS
3. Click on the Exchange mail account and nuke it with the "-".
4. Wait a few seconds
5. Now click the "plus" sign and add your exchange account again.
6. Open up apple mail.
7. Do not add a rule to move messages to different folders. This is what causes the problem.

I suggest anyone wanting to clean up their mailbox, use "Smart Mailboxes" and "Smart Folders" instead of rules. These actually don't get synced to the mail server and only clean up the view on your side.

Hopefully Microsoft and Apple will fix this soon.

A SOLUTION

Goto ~/Library

Containers Folder

++ goto com.apple.com

++ SelectAll

++ goto gear icon, --> New Folder With Selection

Applications Support Folder:

++ goto AddressBook

++ delete anything that looks like "Mail Recent"

Preferences Folder

++ look for com.apple.mail

++ DELETE

Mail Folder

++ select all

++ goto gear icon, --> New Folder With Selection

[[RESTART]]

Note: this solution resets APPLE MAIL to a virgin state. IF YOU HAVE ANY FOLDERS in ON MY MAC they will be LOST / DELETED. Save anything in ON MY MAC somewhere safe. You will have to re- add later

Note: this solution is designed for non-POP (IMAP, Exchange etc type accounts). You may suffer data loss is you use POP account.

Good luck!

Go- Library - deleted Mail Folder and added my account and it worked

The current work around is to disable all your rules.

I created a new user called “test” and set up email using my exchange account and this works consistently error free.

the caveat is you would have to move all your user data to the new user account. I haven’t done this as it is a big job and prone to problems if you miss items, another words there is no documented procedure for this. Apple NEEDS TO FIX THIS.