Invalid base64 string for MIME content

Hello Eastrade,

It looks like from your post you are having issues with the error code "Invalid base64 string for MIME content" appearing when using Mail on your device. We can definitely see how that would be an issue and want to help get it resolved. Do you have any rules setup for the mailboxes/accounts that are reporting this error? If so, try deleting those rules and testing for the issue once more: Use rules to manage emails you receive in Mail on Mac

You may also want to check for any custom actions in Mail > Preferences > Junk Mail > Junk Mail Behaviors > Perform Custom Actions. If you have any enabled try disabling them as well to see if the issue continues.

Hopefully that helps! Have a great rest of your day.

Hello Eastrade,

It looks like from your post you are having issues with the error code "Invalid base64 string for MIME content" appearing when using Mail on your device. We can definitely see how that would be an issue and want to help get it resolved. Do you have any rules setup for the mailboxes/accounts that are reporting this error? If so, try deleting those rules and testing for the issue once more: Use rules to manage emails you receive in Mail on Mac

You may also want to check for any custom actions in Mail > Preferences > Junk Mail > Junk Mail Behaviors > Perform Custom Actions. If you have any enabled try disabling them as well to see if the issue continues.

Hopefully that helps! Have a great rest of your day.

This problem has been annoying and frustrating users for over a year and a half or more. I noticed it Spring 2019. It comes and essentially locks up the application. What I have found in my case is that I think it has something to do with being on email distribution lists where the target email is to "to" as in To: [list name]-l@anywhere.edu, not from: [listname]@anywhere.edu. So if i erased all the rules it went away. I have too many emails to not use rules!! I am sure others have the same or similar situation.

It also stopped when I moved from Apple Mail to Microsoft Outlook, but on a microsoft exchange server with an IT that wants as little to do with apple it can get away with other problems surface. So I am back at Apple Mail for a while and thinking about moving to a Windows box for my next computer purchase.

I hope that for once Apple will listen to users having this problem and actually try to fix it.

Thanks.