Invalid base64 string for MIME content.

SOLVED!

Had a few days worth of back and forth with Apple support. Jaclyn and Patrick solved this.

The steps are a bit complicated but it involved deleting part of the Library/Mail that includes the bad code from Exchange accounts. Call Apple and get them to find the notes from my interaction.

I don't believe this is limited to Exchange accounts, it's just that moving/copying an affected message to an Exchange account actually brings up an error. When attempting to move emails to a GMail account they don't actually move, but are no longer in the originating folder and just seem to disappear.

I am trying to copy emails that I restored from a time machine into the cloud. So far I've tried Gmail and Outlook.com. Here's the process I followed:

1. I copied my /Library/Mail folder from time machine to my Downloads folder
2. I deleted my existing /Library/Mail folder
3. I opened the Mail app, which had my Gmail account connected
4. I imported (using File --> Import Mailboxes) the backup from my Downloads folder
5. These emails were imported to a new "imported" folder located "on my mac"
6. I then started moving emails from the imported folder into my Gmail Inbox and Sent folders.

After getting through 1,000 messages I thought to do a count of emails. I had a starting count before I started moving any messages, and found that through the move process over 200 messages had not actually moved to Gmail, but were no longer in the "imported" folder. No error messages were given.

After going back and restoring again, I then tried this with my Outlook.com account. This time I started getting the "Invalid base64 string for MIME content" error, but the same behavior of the email being removed from the local imported folder but not migrated to the server persists.

Also, I do not have any rules setup in the Apple Mail app and I am running the latest MacOS.

Same issue.

The message “EMAIL SUBJECT” could not be moved to the mailbox “Exchange”

Invalid base64 string for MIME content.

I rebuilt the affected mailbox as well as completely deleted and re-added the exchange account. No luck.

Apple has just released Catalina 10.15.2 and there is NO FIX for the "Invalid base64 string for IME content" issue we are all having. We should all call Apple Care and complain and this. This problem affects all users that use the Apple Mail APP for MS Exchange. It must be affecting millions of users and if you call Apple Care most support reps have not even heard of this problem. I've lost a lot of respect for Apple over this issue, I may even switch back to MS Windows 10. AT least my mail would work again. I have a $2500 MacBook Pro that can't even get MS exchange mail.

Since upgrading to Catalina I have a Mail problem. All emails I have in the Apple Mail App that

are received from Microsoft and gmail have this annoying message:

Invalid base64 string for MIME content.

For the first time I'm very disappointed in Apple.

I am having the same issue on two of four machines. All are accessing the same accounts and it seems to only be w/ mail to my inbox on my exchange based account. What is weird is it works on my two MBP's (2015, 2018) but is behaving badly on my two iMac's (both late 2015). I can't imagine what would cause mail to behave differently on a MBP vs an iMac, guessing I just got unlucky on those two machines.

I'm having the same issue also, but in my case it's only for emails I'm currently composing. As though it's trying and failing to save the message to my drafts folder. Although it actually does show up there. Also using Exchange with mac mail in Catalina on my shiny new MBP16.

its a little odd that you would not "share" your findings with others on a forum, dedicated to others experiencing the same issue, you almost sound as though your bragging.... good to hear you fixed your issue though!

Come on Apple! We're now 3 months into this massively annoying and widespread bug. The fix is all I want for Christmas. Clicking to close 20+ popups ever morning when I log in, and constantly throughout the work day is terrible.

Yes, its getting very old. Strange that I have no problem with iOS, just Catalina. I have tried logging out of the account, deleting everything and then reenabling, but no better. I have complained to Apple directly a couple of times. Does anyone have any suggestions of another email application? I use to use Outlook for work, but its way more than I need for home use.

I purchased a 16" MacBook Pro last November and have been dealing with this issue since, it actually angers me that I can spend £4000.00 for a laptop and have had to tolerate this bug since the release of Catalina, I also have the 2019 15" MacBook Pro and have left this on Mojave... because Catalina is just so unreliable...

How can something so fundamental as the Mail App, be so totally screwed up, if there were other realistic alternatives to the debacle of Mail I would have moved on to one of them, but macOS has many limitations for similar Mail apps, like the lack of spotlight search for Airmail... how can this bug still be here 6 months later!!!!!!!!!!!!!!!!!!

same here! help! Apple!

I also have same problem

Can you post them to save us a lengthy phone call?

So thousands of us need to call support to get this supposed fix?