Fix Error Invalid Base64 String for MIME Content on macOS Catalina

Many Mac users have reported that they are receiving Error “Invalid Base64 String for MIME Content” after upgrading Mac to macOS Catalina. Today in this article I am going to provide few workarounds to fix error “Invalid base64 string for MIME Content” on macOS Catalina. Let’s start with a few practical scenarios.

Scenario 1: Invalid base64 string for MIME content

Since upgrading to Catalina, I keep getting this error with each email that comes in. “Invalid base64 string for MIME content.”. This is really annoying and making me regret doing this upgrade. Does anyone have a fix?

Source: Apple Community

Scenario 2: Email Notification: Invalid base64 string for MIME content

Upgraded to Catalina this morning and now receive email notifications with the code above. It did not happen with Mojave. It only happens to email sent to the email address that uses the Exchange server through Office 365 and not the AOL address. Both are being accessed on a 2014 MacBook Air suing the native Mail app. Any thoughts on how to address this problem? The email gets through, so it is just irritating and not a loss of information.

Invalid Base64 String for MIME Content on Mac

Source: Apple Community

Like the above two users, there are many others who are receiving “Invalid base64 string for MIME content” after Catalina update. Please follow the steps provided below to fix this error.

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Disable and Enable the Mail Services

  • In the Mail app on your Mac, select Mail > Accounts.
  • Select your account, then deselect the Mail tick box.
  • Close your Mail App.
  • Relaunch Mail App and select the Mail tick box again and check for the error message.

Delete Rules in Mail on Your Mac

After intense research, I noticed that deleting the Rules in Apple mail has fixed the error. Few Mac users deleted the Rules in Apple mail and then added the rules again and the error did not occur. So, try deleting the Rules, add the rules again and then check if you are receiving the error or not. In case, the issue still persists then try deleting the Rules but do not add the rules again.

  • Launch the Mail app on your Mac.
  • In the Mail App the select Mail and go to Preferences.
  • Now select the Rules tab.
  • Choose the Mail rule required to delete and then click on Remove.
  • Click on Remove to confirm.
  • Now repeat the steps to remove all the mail rules in case you have multiple rules.
  • Close and open the Mail app again.

How to delete mail rules on Mac

Use Safe Mode

  • Go to the Apple menu and click on Restart.

Apple menu restart

  • Press and hold the Shift key on the keyboard while Mac is restarting.
  • Launch the Mail app and send yourself an email from your personal account to the Exchange Account to check if you receive an error. Most probably you will not receive an error in Safe Mode.
  • Launch System Preference and go to Users and Groups.
  • Choose Login Items.
  • Here delete all the items that are marked Unknown.
  • Now restart your Mac in normal mode.

Remove and Add Email Account

This is to inform you that When the email account is deleted from your Mail app, the email account’s messages are deleted and are no longer available on your Mac. However, the copies of messages continue to remain on the account’s mail server and are still available (from webmail such as Yahoo, Gmail, etc.).

  • In your Mail app, select Mail, go to Preferences and then click on Accounts.
  • Choose your account and then click on the Remove button.
  • Now close the Mail app.
  • To add your account again, open Mail, select Mail > Add Account. Choose your account type, enter all the required necessary account details. Also, ensure that the Mail tick box is checked for your account.

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I hope one of the steps provided in this article helped you Fix Error Invalid Base64 String for MIME Content on macOS Catalina. In case, the issue still persists then please contact Apple Support for further assistance.

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