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How to Resolve “Invalid Base64 string for MIME content’ Error in Apple Mail?

Ritu Roy | email migration fix Software Technology | 5 minutes read | Modified on: 19-12-2024
Invalid Base64 string for MIME content’ Error in Apple Mail

Apple Mail is a popular email client used by many Mac users for sending and receiving emails. While it is generally a reliable platform, users occasionally encounter errors when sending or receiving messages. One such error that can crop up is the ‘Invalid Base64 string for MIME content’ error.

This error typically occurs when the email contains attachments or inline images encoded in Base64 format, and there is an issue with how the encoding was performed or interpreted. Base64 is commonly used for encoding binary data, such as images or files, to be transmitted via email, which is fundamentally text-based. When Apple Mail fails to properly decode or encode these Base64-encoded contents, the result is an error message like ‘Invalid Base64 string for MIME content’.

In this article, we’ll explain what causes this error and guide you through the steps to fix it.

What is the ‘Invalid Base64 String for MIME Content’ Error?

Before we dive into solutions, it’s helpful to understand the technical aspects behind this error:

  • MIME (Multipurpose Internet Mail Extensions): This is the standard for formatting email messages that include text, attachments, and images. MIME allows emails to include various types of media in addition to text.
  • Base64 Encoding: This is a method for encoding binary data into a string of ASCII characters. It is commonly used in emails to encode attachments and images so they can be safely transmitted over text-based email protocols (SMTP).
  • The Error: When Apple Mail cannot properly decode or interpret the Base64-encoded data for MIME content (attachments, images, etc.), you’ll see the ‘Invalid Base64 string for MIME content’ error.

Now that you understand the error’s context, let’s look at how to resolve it.

Common Causes of the Error

  1. Corrupted Email Attachments: If an email’s attachments are incorrectly encoded or corrupted, it may lead to the Base64 error.
  2. MIME Formatting Issues: Sometimes, the problem lies with how the email’s MIME structure is formatted or how it was sent. Email clients or servers may misinterpret or corrupt the MIME structure.
  3. Email Client or Server Bugs: Outdated email clients or misconfigured email servers can lead to this error when sending or receiving emails.
  4. Conflicting Extensions or Add-ons: Third-party apps or extensions installed on your system or email client might be altering how email content is handled, causing encoding issues.

How to Fix the ‘Invalid Base64 String for MIME Content’ Error in Apple Mail

Here are a few troubleshooting steps to resolve the ‘Invalid Base64 string for MIME content’ error in Apple Mail.

1. Update Apple Mail and macOS

The first step in resolving this issue is to make sure that both Apple Mail and your macOS are up-to-date. Sometimes, this error can be caused by bugs in older versions of Apple Mail, which might have trouble interpreting newer MIME or Base64-encoded messages. Apple frequently releases updates to fix bugs, improve security, and enhance compatibility.

To check for updates:

  • Apple Mail Update: Apple Mail/Mac Mail updates are commonly included with macOS updates. To check for updates:
    • Open the App Store.
    • Go to the Updates tab and see if there are any updates for Apple Mail or related system components.
  • macOS Update:
    • Click the Apple Menu > System Preferences > Software Update.
    • If an update is available, click Update Now.

After installing any updates, restart your Mac.

2. Rebuild Mailbox

Apple Mail occasionally experiences issues with corrupted mailboxes, which can cause errors in displaying or decoding MIME content. Rebuilding your mailbox can often resolve these types of issues, including Base64 errors.

To rebuild your mailbox:

  1. Open Apple Mail.
  2. Select the mailbox (Inbox, Sent, etc.) where the error is occurring.
  3. Go to the Mailbox menu at the top of the screen.
  4. Click Rebuild.

This will re-sync your mailbox with the email server, and it may fix any corruption causing the error.

3. Disable Third-Party Email Add-ons

If you have third-party add-ons or email management apps installed on your Mac, one of them might be interfering with Apple Mail’s ability to correctly encode or decode MIME content. To rule out this possibility, try disabling any extensions or third-party apps that modify email behavior, such as spam filters, antivirus software, or email optimization tools.

To do this:

  • Open System Preferences.
  • Go to Extensions or the Security & Privacy section and look for any third-party apps or extensions.
  • Turn them off one by one and see whether the problem still exists.

4. Launch the Mac in Safe Mode.

Safe Mode is an option for starting your Mac. It’s helpful to determine whether the problem you’re having is related to a program that loads when the Mac boots up. Follow these steps to put your Mac (with Apple silicon) in safe mode:

  • Hit Shut Down from the Apple menu on your Mac. Await the complete shutdown of the system.
  • To access the “Loading startup options” screen, click and hold the Power button.  
  • Select the volume.
  • Press the Shift key and hold it.
  • After that, select “Continue in Safe Mode.”
  • The menu bar will display “Safe Boot” and the system will restart on its own.

5. Disable/Enable Mail Services

If there is an issue with your email account, you can see the error “Invalid Base64 string for MIME Content.” In that scenario, you can turn off the email account for a bit, and then switch it back on. To do so, follow these steps:

  • On your Mac, launch the Mail application and choose Mail > Accounts.
  • Once the mail account has been selected, deselect the Mail checkbox.  
  • Exit the program.
  • Launch the Mail app now, then choose the Mail check box once more.
  • Verify that the error has been fixed.

Author Suggestion!

The “Invalid Base64 string for MME Content” error can be fixed using the techniques listed above. It is recommended that you move your Mac Mail emails to Microsoft Outlook if the error recurs or if you are having problems accessing your emails on Mac Mail. However, the data file formats used by the two email clients are different. While Outlook utilizes PST files, Mac Mail uses MBOX files. Therefore, converting MBOX files into Outlook-compatible PST files is necessary when importing or transferring emails and other data from Mac Mail into Outlook.

However, there are numerous steps involved, and the manual mail transfer process from Apple Mail to Outlook will take a long time. Using a professional MBOX to PST Converter application, like CubexSoft MBOX Converter, is a smooth method of completing the migration process from Apple Mail to Outlook. The program can quickly convert Mac Mail MBOX files to Outlook PST files that can be imported. The program makes it simple and quick to move all of your contacts, emails, attachments, and other mailbox contents from Apple Mail to Microsoft Outlook.