Mozilla Thunderbird, a cross-platform email client, news client, RSS reader, and chat client, is freely available as open-source software. The Mozilla Foundation is responsible for its development, and it employs the XUL interface language. Additionally, Mozilla Thunderbird comes pre-installed in Ubuntu desktop systems as the default email client.

The MBOX Format

MBOX is a file format designed for consolidating a vast collection of emails, encompassing both sent and received messages, typically identified by the .mbox extension. This format enables the aggregation of all mailbox messages into a single document, facilitating convenient access to each individual email.

This file format stores emails sequentially, with each email following the previous one. While initially designed for Unix systems, it has gained compatibility with major email applications like Mozilla Thunderbird, Apple Mail, and email clients such as Gmail over time.

Reasons to Migrate MBOX to Office 365
  1. To unlock further features within Office 365,
  2. The MBOX file cannot load emails.
  3. The MBOX file is of considerable size and cannot be managed manually.
  4. Encountering recurring errors within the MBOX files.
Methods to Migrate Thunderbird MBOX to Office 365

Presently, two distinct approaches are available for transferring MBOX data to Office 365.

Automated tool to migrate Thunderbird to Office 365

There is a way by which migration from Thunderbird MBOX to Office 365 file can be performed quite efficaciously. This is by using a third-party migration tool like Kernel MBOX to Office 365 Migrator. The migration process can be effortlessly completed through a simple three-step procedure. Below, we elucidate the individual steps involved.

  1. You can either search for the directory containing MBOX files or import the MBOX files themselves. Brows folder containing MBOX files
  2. During this stage, you have the opportunity to review the mailbox data. Following the preview, choose the 'Office 365' option to store the data. Select Office 365 option to save file
  3. Please furnish your Office 365 login details and securely store the MBOX data. Provide Office 365 credentials

This tool has been meticulously crafted to surmount any obstacles encountered during the migration from Thunderbird to Office 365. It operates seamlessly without any technical hitches, ensuring swift and hassle-free export of Thunderbird MBOX files to Office 365.

Native method to migrate Thunderbird to Office 365

There are various factors driving users to transition from MBOX to Office 365, and yet, a considerable number may not be acquainted with the straightforward and user-friendly approaches at their disposal. Some of these methods are uncomplicated and demand no specialized technical expertise. With these methods, you can effortlessly migrate from MBOX to Office 365 without experiencing any data loss.

Converting MBOX files to Office 365 is a straightforward process that can be accomplished in just two simple steps. Firstly, use Kernel for MBOX to PST Converter tool to convert MBOX files to PST files. Secondly, import PST files to Office 365 mailbox manually using native methods.

Kernel MBOX to PST home screen

If we want it to be stored in Office 365:

  1. Access
  2. In the left panel select Installation> Data migration Select Installation> Data migration
  3. Choose to upload PST files. Select Upload PST files
  4. Choose a new import task. Select New import job
  5. Name our project and then select Next. Set a name for our work
  6. Choose "Load data" and then proceed by clicking on "next." Select Load data
  7. Choose step 2 (Enable SAS network's upload URL display). Select point 2
  8. "Please obtain and set up the Azure AzCopy tool by following the instructions in step 3."
  9. We run the Microsoft Azure Storage AzCopy tool We run the Microsoft Azure Storage AzCopy tool
  10. Please proceed by entering the subsequent command:

    AzCopy.exe /Source:<location of="" pst="" files=""> /Dest:<sas url=""> /V:<log file="" location=""> /Y /Soruce: “directory where the PST file is located” /Dest: “the SA URL that was obtained in step 2”</log></sas></location>

  11. Here's an illustration of the expected parameter format:

    AzCopy.exe /Source:"\\FILESERVER1\PSTs" /Dest:"" /V:"c:\Users\Admin\Desktop\AzCopy1.log" /Y

  12. The commencement of PST file upload is underway. Upload PST file
  13. After the upload has finished, it will display the following status:

    Status of uploded file
The subsequent action involves generating a CSV file that links the email account to the PST file.
  1. Launch Microsoft Excel and paste the provided content into a new document. After pasting the content, save the file in CSV format.
  2. Workload,FilePath,Name,Mailbox,IsArchive,TargetRootFolder,ContentCodePage,SPFileContainer,SPManifestContainer,SPSiteUrl Exchange,,annb.pst,,FALSE,/,,,, Exchange,,annb_archive.pst,,TRUE,,,,, Exchange,,donh.pst,,FALSE,/,,,, Exchange,,donh_archive.pst,,TRUE,,,,, Exchange,PSTFiles,pilarp.pst,,FALSE,/,,,, Exchange,PSTFiles,pilarp_archive.pst,,TRUE,/ImportedPst,,,, Exchange,PSTFiles,tonyk.pst,,FALSE,,,,, Exchange,PSTFiles,tonyk_archive.pst,,TRUE,/ImportedPst,,,, Exchange,PSTFiles,zrinkam.pst,,FALSE,,,,, Exchange,PSTFiles,zrinkam_archive.pst,,TRUE,/ImportedPst,,,,

    The first row or header row of the CSV file lists the parameters that will be used in the PST import service to import the PST files into the user mailboxes.

  3. Use the information in the following table to fill in the CSV file with the necessary information.
  4. Parameter Description Example
    Workload Indicate the Office 365 service for importing data by selecting Exchange, which allows the import of PST files into users' mailboxes. Exchange

    Indicates the precise Azure storage directory where the PST files were loaded during the process described in step 2. In step 2, when configuring the URL for SA (Subfolder Application), handling the optional subfolder name in the /Dest parameter depends on whether you include it or not. If you omit the subfolder name, please ensure that the CSV file leaves this parameter empty. Conversely, if you include the subfolder name, make sure to specify it within this parameter as shown in the second example. Please note that the value of this parameter is sensitive to letter casing.

    In both instances, "ingestiondata" is not included as part of the value for the FilePath parameter.

    Crucial Note: It is imperative that the letter case in the file path name aligns precisely with the case used when specifying an optional subfolder's name within the SA URL in the "/ Dest" parameter during step 2. To illustrate, if you designate "PSTFiles" as the subfolder name in step 2 and subsequently employ "pstfiles" in the FilePath parameter within the CSV file, the PST file import will encounter failure. It is essential to maintain consistency in letter case in both instances.

    (left blank)

    O well: PSTFiles


    Indicates the specific PST file name for import into the user's mailbox. It's important to note that the value for this parameter is sensitive to letter casing.

    Crucial Note: The letter case of the PST file name specified in the CSV file must precisely match the case of the PST file loaded into the Azure storage location during step 2. To illustrate, if you employ "annb.pst" in the Name parameter within the CSV file, yet the real PST file's name is "AnnB.pst," the PST file will not be imported. It is imperative to ensure that the PST file's name in the CSV file is in the exact case as the genuine PST file.


    Indicate the email address associated with the mailbox you wish to import into the PST file. Please be aware that it's not possible to designate a public folder, as the PST file import service does not have the capability to import PST files into public folders.

    In order to import a PST file into a dormant mailbox, it is essential to provide the mailbox GUID as a parameter. To acquire this GUID, execute the following PowerShell command within Exchange Online: Run "Get-Mailbox -InactiveMailboxOnly | FL Guid."

    Note: Sometimes, you might find yourself in a situation where you have multiple mailboxes associated with a single email address. Among these mailboxes, one may be considered an asset, while another could be in a soft-deleted or inactive state. In these cases, you have to specify the GUID mailbox to identify the mailbox to import the PST file. To obtain this GUID of asset mailboxes, run the following PowerShell command: Get-Mailbox <identity of active mailbox> | FL Guid. To get the GUID of soft (or inactive) deleted mailboxes, run this command Get-Mailbox <identity of soft-deleted or inactive mailbox> -SoftDeletedMailbox | FL Guid. O well:



    Indicates your preference for importing the PST file into the user's file mailbox. There are two available choices:

    Incorrectly imports the PST file into the user's main mailbox.

    Incorrectly importing the PST file into the user's mailbox is implied here. It assumes the user's mailbox is active. When you designate this parameter as TRUE and the user's mailbox is not active, the import process will be unsuccessful for that specific user. It's important to recognize that if an import fails for a user (due to the mailbox not being active and this attribute being set to TRUE), it won't impact the other users in the import task. If you choose to omit this parameter, the PST file will be imported into the user's main mailbox.

    Note: To import a PST file into a cloud-based mailbox for a user whose primary mailbox is local, simply set the parameter to TRUE and provide the email address associated with the user's local mailbox as the value for the Mailbox parameter.


    O well:



    Indicates the destination mailbox folder for importing the PST file.

    If you omit specifying this parameter, the PST file will be brought into a fresh folder with the imported name. This folder will be positioned at the root level of the mailbox, alongside default folders like the Inbox and other mailbox folders.

    (left blank)
    O well/ O well:


    This optional parameter allows you to define a numerical value for the code page when importing PST files in the ANSI file format. Its purpose is to facilitate the import of PST files originating from Chinese, Japanese, and Korean (CJK) organizations, as these languages often employ a double-byte character set (DBCS) for character encoding. Without using this parameter for importing PST files associated with languages that utilize DBCS for mailbox folder names, there's a common issue of folder names becoming blurry upon import.

    To view a comprehensive list of acceptable values for this parameter, please refer to the Code Page Identifiers documentation. Note: As mentioned earlier, this parameter is entirely optional and does not require inclusion in the CSV file. Alternatively, you have the choice to include it while leaving the value empty for one or more rows.

    (left blank)

    O well

    932 (which is the identifier of the Japanese ANSI / OEM code page)

    SPFileContainer To import PST files, keep this parameter empty. Not applicable
    SPManifestContainer To import PST files, keep this parameter empty. Not applicable
    SPSiteUrl To import PST files, simply leave this parameter empty. Not applicable

  5. In the Office 365 administration portal, please mark the checkboxes and then proceed by selecting "next." Office 365 administration portal
  6. Choose the CSV file, perform validation, and then save it. validate CSF file and save

After completing the analysis of the .pst file's data, you may proceed with the import process.

  1. Click on Are you ready to import to Office 365? Start import to Office 365 process
  2. Click on Import to Office 365 located on the floating page. When you access the page, it will display a filtering interface for your data. This interface presents data impressions generated from the analysis performed on PST files within Office 365, including information about the data's age. At this point, you can choose to either filter and import specific data or import all the data as it is.
  3. Choose No, I prefer importing all items. No, I want to import everything
  4. Choose the Import data option to initiate the data import process.

Final Word

When transitioning to Office 365, the need to migrate MBOX files arises in order to access our personal emails within the new Office 365 mailbox. However, accomplishing this task smoothly, without encountering conflicts, can be challenging, particularly if you are unfamiliar with the required techniques. In this guide, we aim to provide a comprehensive overview of the various methods available for efficiently migrating MBOX files to Office 365.