How to Import MBOX into Outlook 365 in 3 Ways

IT admins and organizations who want to adopt the M365 cloud need to know how to import MBOX into Outlook 365. We present multiple options to choose from. Before anything else, let us explain why a simple drag-and-drop type import that works for many other file types fails for MBOX.

Why You Can’t Just Drag-and-Drop or Rename MBOX Files for Outlook 365?

When trying to get MBOX files into Outlook 365, users may try dragging and dropping or renaming files to make them Office and Microsoft acceptable. Unfortunately, it doesn’t work and can even cause problems. This limitation is widespread across the Outlook ecosystem.

Moreover, any form of “success” in even “placing” MBOX files via drag-and-drop is largely limited to the Classic Outlook desktop version. Across all other Outlook platforms, this method is generally ineffective or not possible. See:

• New Outlook Desktop Client: Drag-and-drop of MBOX files is largely disabled or unresponsive. Moreover, if you drag MBOX into the main inbox folder, Outlook treats it as an attachment and drafts up a new email.
• Outlook 365 Webmail (Outlook on the Web): Similar to the New Outlook client, drag-and-drop functionality for importing MBOX files is not supported in the Office 365 webmail interface.
• Outlook Mobile Applications (iOS/Android): Mobile email applications do not have options or interfaces for direct MBOX file imports or drag-and-drop of any kind.

What Happens If You Try to Import MBOX into Outlook 365 Classic Edition with a Cursor?

• Into your Inbox (Classic Outlook): If you try to drag and drop an MBOX file directly into your Outlook Inbox in the classic client, Outlook typically thinks you’re trying to add an attachment to a new email.
  As a result, it will launch a new mail draft and place the MBOX file there as an attachment. This isn’t importing the emails within the MBOX file; it’s just attaching the entire MBOX file itself. We don’t want that.
• Into other default folders (Drafts, Sent Items – Classic Outlook): In our testing with Classic Outlook, we were able to add an MBOX file to folders like Drafts and Sent Items. However, it remains largely unusable.
  The MBOX file just sits there, essentially like any other text file. You can’t forward individual emails from it, transfer them between folders as distinct messages, etc.
• Custom Folders (Classic Outlook & Syncing to New Outlook): Initially, folders created or MBOX files added in default folders like Drafts or Sent Items in Classic Outlook might not immediately appear in the New Outlook client until a sync occurs (i.e., going to the View tab and pressing “Sync”). However, this doesn’t mean it’s usable.
• Into Outlook Classic’s Default Data File Folder: Adding an MBOX file directly into the root “Outlook Data File” folder in classic Outlook versions results in an error. As Outlook treats it as a potentially unsafe item.
• The “New Outlook” Client Experience (for synced MBOX files): Even for MBOX files that have synced across from Classic Outlook (e.g., in a custom folder), there’s a glaring issue in the New Outlook client: the MBOX files will not show any preview whatsoever. If you double-click them to try and open them, they simply download to your computer as an .mbox file again. This is definitely not what we want.

Consequences of Improperly “Adding” MBOX Files into Outlook 365

Even if you manage to get the MBOX file to sit in an Outlook folder, the content is not truly imported or usable as individual emails:

• Attachments are missing: The attachments from the original emails within the MBOX file will not be accessible.
• Emails are clumped together: All the emails from the MBOX file will appear as one single block of data, not as separate, manageable messages.
• Metadata is merged with actual mail content: Important information like sender, recipient, date, and subject (the metadata) gets jumbled together with the body of the emails, making it unreadable and disorganized.

There are other DIY solutions that make it possible to use MBOX archives from Google Takeout, old Thunderbird profiles, and personal backups. Let us see what they are.

Append Office 365 Account in Thunderbird and Add MBOX Files

If you plan to use Thunderbird as an intermediary platform, you have two different routes. First is add your Office 365 business, Microsoft 365 enterprise account to the Thunderbird platform, then use the import feature to add the MBOX files directly to your Office account, bypassing the Outlook client requirement.

The steps are as follows:

Part 1. Adding an Office 365/Microsoft 365 Account to Thunderbird

Before you begin, visit the Thunderbird settings and make sure you allow cookies from sites. This is important for modern OAuth2 functionality.

Open Thunderbird, go to File > New > Email Account.

Enter your full name, email address, and password when prompted.

Thunderbird should automatically configure the Incoming Server data; verify these values:

• Protocol: IMAP
• Server hostname: outlook.office365.com
• Port: 993
• Connection security: SSL/TLS
• Authentication method: OAuth2
• Username: Your complete email address

Same for the Outgoing Server:

• Server hostname: smtp.office365.com
• Port: 587
• Connection security: STARTTLS
• Authentication method: OAuth2
• Username: Your complete email address

You’ll be directed to a Microsoft web page to finalize the login and grant permissions. Follow those steps.

Part 2. Importing MBOX Files to Thunderbird

• Step 1. Go to Settings > Add-ons and Themes > Switch to Extensions and Search for ImportExportTools NG.
• Step 2. Click on Add to Thunderbird, then Add. Thunderbird will add the extension.
• Step 3. Back in the main Thunderbird menu, right-click on “your account” and create a new folder specifically for your imported emails.
• Step 4. Right-click the new folder you created, hover over “ImportExportTools NG,” and then select the option to import MBOX files.
• Step 5. Browse to where your MBOX files are saved, select them, and begin the import.

Use Thunderbird to Make MBOX Office 365 Compliant

The second option is to add the MBOX file(s) into an account-free Thunderbird client. Download it again, this time in EML format. Then use the new Outlook 365 client’s .eml import feature to bring the now converted MBOX into the Office 365 account.

Here is how you would do it:

Part 1: Before you import MBOX files into your Outlook 365 Account, convert them via Thunderbird.

• Step 1. Download and install Thunderbird (Admins need to do it for every user individually).
• Step 2. Open Thunderbird, right-click on the Local Folders > New Folder.
• Step 3. Name it (e.g., Temporary MBOX storage) and hit Create Folder.
• Step 4. Go to Settings > Add-ons and Themes > Switch to Extensions and Search for ImportExportTools NG.
• Step 5. Click on Add to Thunderbird, then Add. Thunderbird will add the extension.
• Step 6. After that, go back to the main Thunderbird screen. Right-click on the local subfolder you made earlier and hover over “ImportExportTools NG,”.
• Step 7. Then, select the option to import MBOX files.
• Step 8. Choose any one of the four options (individual, directory | with/without sbd) that works best for your scenario.
• Step 9. Use the browse window to get the MBOX and click Open. Soon, you will see all the MBOX files inside the Local subfolder.
• Step 10. After that, go back to the main Thunderbird screen. Right-click on the local sub-folder with MBOX files and hover over “ImportExportTools NG,”.
• Step 11. This time select Export All Messages In Folder > EML Message format > Messages and Index with subfolder.
• Step 12. Browse for a location, hit Select Folder, and let Thunderbird do its thing.

Part 2: Add the now converted MBOX into an Outlook Client with a Microsoft 365 Account

We are using the Import mechanism of the New Outlook Client to add the data.

• Step 1. Open Outlook, click on the settings cog > go to File > Import > Click Start Import.
• Step 2. Browse for the folder containing converted MBOX files (now they are true .eml files).
• Step 3. Use the Import email dialog box to choose a destination, i.e., Account and Folder (both have dropdown menus).
• Step 4. Once done, click Import.
• Step 5. You should see a real-time upload count with a progress bar. Once it completes, go to the Outlook 365 folder that you selected and check for the emails.

All of this sounds like too much of a hassle because it is. However, this does not mean you should try to cheat out a way with a simple file extension rename. Let us see why.

Renaming MBOX File Extensions to Make it Outlook 365 Compatible is Problematic

Please do not attempt to manually change the .mbox extension to .eml (or add the .eml extension to extension-less MBOX files). This won’t make your MBOX file Outlook compatible (on any Outlook platform). Moreover, it adds the risk of corrupting the data. It’s far better to take the lengthier but safer approach of proper conversion.

The same warning applies to those who might think they can make their MBOX file ready for PST import (for the classic Outlook client) by simply renaming the extension to .pst.

Why These “Shortcuts” Fail: The Importance of Internal Data Structure

There is a simple reason that changing file extensions doesn’t work: Changing a file extension will not change its internal data structure. Moreover, this internal structure remains different in MBOX, EML, and PST even after the extension gets renamed. So, Outlook 365 or any other client, for that matter, may accept it for import but won’t render it correctly. You can avoid all the trouble by simply choosing the right solution.

Admins can mass convert MBOX files into the Outlook 365 format and import

Use the MBOX conversion tool to actually change the .mbox file type into .pst. Then use any of the import options, like Drive Shipping or Network Upload, to get this data inside the user accounts.

There is an even better and more direct solution. Plus, you can check it out right here, right now.

Conclusion

Now, readers know exactly what to do when they have a query like how to import MBOX into Outlook 365. Here, we explained why the MBOX file was incompatible with the built-in import mechanism of Outlook 365 clients, increasing the difficulty of adding mailbox data into the Office 365 accounts.

Individual users who have more freedom can attempt a self-transition via the Thunderbird client, but this becomes quite cumbersome for businesses using Office 365. That is why we also gave instructions on how to use a utility that is a sure-shot way to bring in the MBOX files into the cloud platform in professional environments.