Email Norification: Invalid base64 string for MIME content

same here and an added error.....mail WITH attachment report the error message but vanish into nothing....not even paper basket...

Very happy if “turning off RULES” works for you. It didn’t for me. Hence the HEAVY LIFTING METHOD above

Result: no more errors AND all rules activated

A few rules not working is not a big problem for me, I’ll take my chances by just disabling my rules. For folks that have many rules then your way would be best. How did you even figure that out?

I'm having the same issue. It moves the message to the proper folder, but reports that it can't. It looks like it's only happening with one sender. I reported this to Apple. I suppose you can do the same The more people who bug them about it (no pun intended), the faster they'll fix it.

I'm having the same issue with Exchange. It seems the sent mail message first goes to the On My Mac Outbox and the error occurs when transferring it to Exchange. It sits for a minute or so, then I receive the error although the email does send.

Exactly the same message on receiving email through Exchange accounts (not other types of email account). One was a personal email account on Office365 and the other work email account also Office365 - both display display this message on receipt of every email (but the email is still reaches the inbox and is able to be moved to other mailboxes thereafter):

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 (and then emptied the bin of the several thousand files now in it - but don't know whether that was necessary).

When I opened Mail, it re-downloaded the folders and messages again (takes ages) and when I receive messages (so far), I no longer get the error message.

Seems to have worked for me... so far. But as I said, don't know whether I've affected anything else negatively by doing it.

To update, I’ve not had a recurrence of the error message since deleting the whole Mail library folder (described above), and it appears to have had no ill-effect on the system. Not claiming any global ‘fix’ but can report that it has so far worked for me.

1- Go in Finder

2- Click onMacbook

3- Click on Go on the menu

4- Go down and click on Go to

5- Type in: ~/Library/ and enter

6- Click on Mail

7- Delete folder

You'll have to put back your email accounts

Sorry, I'm not very good I explaining but it worked. Hope this will help you.

Yes, this is a workaround but is NOT a fix. This is why creating a new user login works. When you create a new user and open the mail APP there are no rules setup. So now I have to continually look into my spam/junk folder.

I found a solution (for me).

I went to my internet accounts, unticked apple mail from my exchange account, restarted my Mac and then went in and ticked the box again.

It previously was in a seemingly never-ending loop and I could not do anything in Apple mail.

The other troubleshooting steps were unattractive because they entailed turning off RULES.

That was a complete non-starter: my workflow depends on rules to deal with rhe 3,500+ messages per month that I get across accounts

And, after nearly a week, Apple Mail — with all RULES on — has remained stable

Sidenote: I downloaded files from a dodgy source & ran CLAMXAV to check them. Interestingly it flagged malware attached to Apple PhotoLibrary. Name: xxx.Base64.xxxx-xxxx

I have no idea whether the wonkiness in AppleMail and this event are related.

Deleted Apple PhotoLibrary and redownloading a backup from ICloud

Hitherto AppleMail config remains stable

Likewise … Catalina + Apple Mail + Exchange Account = “Invalid base64 string for MIME content."

I tried a Mailbox Rebuild, but no change in behaviour !

This looks one for Cupertino ...

Under the Mail / Accounts menu, I have selected the Exchange account, unchecked the Mail service, restarted the mail app, and switched the service back again. It seems the error is gone now.

Have taken much more extensive action than this (but including this) and none have worked. Neither has deleting the account across my Macs and then adding it again.