Mozilla Thunderbird is a free and open source cross-platform email client, news client, RSS and chat client developed by the Mozilla Foundation. It uses the XUL interface language and is installed by default in Ubuntu desktop systems.

The MBOX Format

MBOX is a format to store a large number of emails in a single file, both sent and received messages, and use the .mbox extension. With it, all the messages of a mailbox can be concatenated in a single document, which will allow you to access each of the emails.

This type of files stores the mails in a concatenated way, so that each one is stored after the other. It is a format that was originally created for Unix systems, but over time has been supported by the main mail applications, such as Mozilla Thunderbird and the Apple Mail program or clients like Gmail.

Reasons to Migrate MBOX to Office 365
  1. To access additional functionality of Office 365
  2. The MBOX file is unable to load emails
  3. The MBOX file size is too large to be handled manually
  4. Facing error again and again in the MBOX files
Methods to Migrate Thunderbird MBOX to Office 365

Now, there are two distinctive methods that can be used to migrate 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 can be smoothly accomplished in just three steps. The steps involved are explained below:

  1. Browse for the folder containing MBOX files or add the MBOX files. Brows folder containing MBOX files
  2. In this step, you can preview the mailbox data. After the preview, select the option ‘Office 365’ to save the data. Select Office 365 option to save file
  3. Provide the Office 365 credentials and save the MBOX data. Provide Office 365 credentials

The tool has been designed to overcome all the impediments faced during the Thunderbird to Office 365 migration. This tool is free from any technical glitches and instantly exports Thunderbird MBOX to Office 365.

Native method to migrate Thunderbird to Office 365

There could be multiple reasons behind the scenes that users are switching from MBOX to Office 365. But, many of you out there are not aware of the simple and easy methods that are available for it. Some of the methods are very simple to perform and require no technical skills. You can complete MBOX to Office 365 conversion without any loss of data.

One can easily convert MBOX files to Office 365 in two 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.

If we want it to be stored in Office 365:

  1. Access
  2. In the left panel select Installation> Data migration
  3. Select Upload PST files
  4. Select New import job
  5. Set a name for our work and click on Next
  6. Select Load data and click on next
  7. Select point 2 (Display upload URL in the SAS network)
  8. Download and install the tool from point 3 Azure AzCopy
  9. We run the Microsoft Azure Storage AzCopy tool
  10. Execute the following 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. Example of how the parameter should look:

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

  12. Upload of the PST file will begin Upload PST file
  13. Once the upload is complete, it will show the following status:

    Status of uploded file
The next steps is to create the CSV file that associates the mail account with the PST file:
  1. Open an Excel and paste the following content, once paste save the file with 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 Specifies the Office 365 service in which the data will be imported. Use Exchange to import PST files into users' mailboxes. Exchange

    Specifies the location of the folder in the Azure storage location that has loaded the PST files a in step 2. If you do not include a name of the optional subfolder in the URL of SA in the / Dest parameter: in step 2, leave this parameter blank in the CSV file. If you include a name of the subfolder, specify in this parameter (see the second example). The value of this parameter is case-sensitive.

    In both cases, they do not include "ingestiondata" in the value of the FilePath parameter.

    Important: The case of the file path name must be the same as the case you use if you include a name of the optional subfolder in the SA URL in the / Dest parameter: in step 2. For example, If you use PSTFiles for the name of the subfolder in step 2, and then use pstfiles in the FilePath parameter CSV file, the import for the PST file will fail. Be sure to use the same case in both cases.

    (left blank)

    O well: PSTFiles


    Specifies the name of the PST file that will be imported into the user's mailbox. The value of this parameter is case-sensitive.

    Important: The case of the PST file name in the CSV file must be the same as the PST file that was loaded in the Azure storage location in step 2. For example, if you use annb.pst in the Name parameter in the CSV file, but the name of the actual PST file is AnnB.pst, the PST file will be imported. Make sure that the name of the PST in the CSV file uses the same case as the actual PST file.


    Specifies the email address of the mailbox to be imported into the PST file. Note that you can not specify a public folder because the PST file import service does not support importing PST files into public folders.

    To import a PST file into an inactive mailbox, you have to specify the mailbox GUID for this parameter. To obtain this GUID, run the following PowerShell command in Exchange Online: Get-Mailbox <identity of inactive mailbox> -InactiveMailboxOnly | FL Guid

    Note: In some cases, you may have several mailboxes with the same email address, where one mailbox is an asset and another mailbox is in a soft (or inactive) deleted 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:



    Specifies whether or not you want to import the PST file into the user's file mailbox. There are two options:

    FALSE Imports the PST file into the user's primary mailbox.

    FALSE Imports the PST file into the user's file mailbox. This assumes that the user's file mailbox is enabled. If you set this parameter to TRUE and the user's file mailbox is not enabled, the import will fail for that user. Note that if an import fails for a user (because your file is not enabled and this property is set to TRUE), the other users in the import job will not be affected. If you leave this parameter blank, the PST file is imported into the user's primary mailbox.

    Note: To import a PST file into a cloud-based file mailbox for a user whose primary mailbox is local, just specify TRUE for this parameter and specify the email address for the user's local mailbox for the parameter Mailbox


    O well:



    Specifies the mailbox folder that the PST file is imported into.

    If you leave this parameter blank, the PST file will be imported into a new folder with imported name located at the root level of the mailbox (the same level as the Inbox folder and the other folders in the mailbox by default).

    (left blank)
    O well/ O well:


    This optional parameter specifies a numeric value for the code page when importing PST files in the ANSI file format. This parameter is used to import PST files from the Chinese, Japanese and Korean (CJK) organizations because these languages often use a double-byte character set (DBCS) for character encoding. If this parameter is not used to import PST files for languages that use DBCS for mailbox folder names, folder names are frequently blurred once they are imported.

    For a list of supported values for this parameter, see Code page identifiers. Note: Previously indicated, this is an optional parameter and does not have to be included in the CSV file. Or, you can include it and leave the value blank 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, leave this parameter blank. Not applicable
    SPManifestContainer To import PST files, leave this parameter blank. Not applicable
    SPSiteUrl To import PST files, leave this parameter blank. Not applicable

  5. In the Office 365 administration portal, check the boxes and click on next Office 365 administration portal
  6. Select the CSV file, validate and save. validate CSF file and save

Once the analysis of the data of the .pst file is done, you can start the import:

  1. Click on Are you ready to import to Office 365? Start import to Office 365 process
  2. On the floating page, click Import to Office 365. The page will open to filter the data. It contains the data impressions resulting from the analysis made in the PST files by Office 365, including the information on the antiquity of the data of. At this time, you have the option to filter the data that will be imported or import all the data as is.
  3. Select No, I want to import everything No, I want to import everything
  4. Select Import data and the import of the data will begin.

Final Word

Migrating MBOX to Office 365 becomes a necessity when we want to access our personal emails in our new Office 365 mailbox. But, exporting a huge amount of data without any conflicts is not easy, especially when you’re not aware of the techniques. So, here we have tried to cover every possible method that can be used to migrate MBOX files to Office 365 quickly.