How to Fix ‘Invalid Base64 string for MIME content’ Error in Apple Mail?

Several Apple Mail (Mac Mail) users have reported encountering the error ‘Invalid Base64 string for MIME Content”, after upgrading their Mac OS. Some users have also encountered this error after activating or creating the Rules. Due to this error, they’re facing issues while moving messages and downloading emails. If you’re one such user and facing this error, you can follow the solutions mentioned below to resolve the error.

Methods to Fix ‘Invalid Base64 string for MIME Content’ Error in Apple Mail

Sometimes, a simple restart of the application and system can resolve the issue. You can restart the Apple Mail application and the system, and see if it resolves the error. If not, then follow the below solutions to resolve the error.

Method 1: Disable/Enable Mail Services

You may encounter the ‘Invalid Base64 string for MIME Content” error if there is a problem with your email account. If this is the case, you can temporarily disable the email account and then enable it. Here are the steps to do so:

1. Open the Mail app on your Mac Machine and select Mail > Accounts.
2. After selecting the mail account, deselect the Mail tick box.
3. Close your application.
4. Now start your Mail app and select the Mail tick box again.
5. Check if the error is resolved.

Method 2: Delete Rules in Mac Mail

Several users were able to resolve the error after deleting the Rules in Mac Mail. You can delete the rules in Mac Mail and then see if the error persists. Here are the steps you can follow to delete the Rules:

• Start the Mac Mail application.
• Then select Mail and go to Preferences.
• Open the Rules tab and delete the required Rules.
• Then click on Remove and confirm.
• Close the Mail app and open it again.  

Method 3: Start Mac in Safe Mode

You can start your Mac in Safe Mode. It helps to find out if the issue you’re encountering is caused by a software that loads when the Mac starts up. To start Mac (with Apple silicon) in safe mode, follow these steps:

• On Mac, go to Apple menu and click Shut Down. Wait for system to shut down completely.
• Now press and hold the Power button until the “Loading startup options” appears.
• Choose a volume.
• Press and hold the Shift key.
• Then, click Continue in Safe Mode.
• The system will restart automatically and “Safe Boot” appears in the menu bar.

Method 4: Check and Update your Mac

If the error has occurred due to any bug or issue in your macOS, then you can install the latest updates for your macOS. It may help fix the issue. Here are the steps to check and install updates:

• On the Apple menu, click on System Settings.
• Click on General and then Software Update (You may also choose System Preferences and then click on Software Update).
• If any update is available, click on Update Now or Upgrade Now button to install it.
• When prompted, enter your administrator credentials.

Note: During the installation process, your Mac might restart and display blank screen several times.

Conclusion

The above given methods will help you fix the ‘Invalid Base64 string for MME Content’ error.  In case the error occurs again or you’re facing trouble in accessing your emails on Mac Mail, then it is advisable to migrate your Mac Mail emails to Microsoft Outlook. However, both email clients use different data file formats. Mac Mail uses MBOX files and Outlook uses PST. Hence, migrating or importing emails and other data from Mac Mail into Outlook requires conversion of MBOX files into Outlook compatible PST files.

However, the process of manually transferring mail from Apple Mail to Outlook will take a lot of time as it involves a lot of steps. A seamless way to perform the migration process from Apple Mail to Outlook is by using a reliable MBOX to PST converter tool, such as Stellar Converter for MBOX. The software can easily convert Mac Mail MBOX files into Outlook importable PST format. The software helps you to migrate all your emails, contacts, attachments, and other mailbox contents easily and quickly from Apple Mail to Microsoft Outlook.

Was this article helpful?

YES0

NO

About The Author

Ravi Singh

Ravi Singh is a Senior Writer at Stellar®. He is an expert Tech Explainer, IoT enthusiast, and a passionate nerd with over 7 years of experience in technical writing. He writes about Microsoft Exchange, Microsoft 365, Email Migration, Linux, Windows, Mac, DIY Tech, and Smart Home. Ravi spends most of his weekends working with IoT (DIY Smart Home) devices and playing Overwatch. He is also a solo traveler who loves hiking and exploring new trails.