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Import messages from Thunderbird to Microsoft Outlook

In this article learn how to import messages from Thunderbird to Microsoft Outlook.

Mozilla Thunderbird:

  • has an uncertain future (read more),
  • there are no feature updates, just security fixes for a long time,
  • it is not the best Mail application on Linux nor on Windows,
  • it lacks some useful features (although some add-ons are supported).

At the enterprise level it is ridiculous to use Thunderbird, especially if you have a licence for Microsoft Outlook. So this article is for you guys who want to migrate from Thunderbird to Microsoft Outlook, and therefore import all messages from Thunderbird to Microsoft Outlook.

Thunderbird still doesn’t have an Export feature which would easily export e-mail messages to Outlook. In this article, we will perform two steps to get everything done:

  1. In the first step, free application IMAPSize will be used to export e-mail messages from Thunderbird in EML format (so every message as a whole has its own file),
  2. The second step implies importing files to Outlook.

The catch is that Mozilla Thunderbird stores emails in the mbox format, and MS Outlook stores messages in *.pst files which are binary proprietary (Microsoft) files. Outlook can import a variety of formats, but it can’t import mbox files.

1. Export e-mail messages from Thunderbird in format

OK, it’s time to download application: [ download ]

Note: although this software is no longer under active development since 2009. it works perfectly on Windows 10, 64-bit. Also note that besides installer, you may download the portable version of the software.

Launch the application, then click on the Tools menu and select mbox2eml… item:

Then click on the Select mbox files to convert button; you might need to set the “Files of type” option to “All Files” in the file selection dialog, since Thunderbird mbox files don’t have an extension.

Afterwards, click on the button to select the destination folder for files.

Finally, click on the Convert button to start the mbox to eml conversion process.

Note: Thunderbird keeps messages in mbox files even after they have been deleted. If you don’t want to include deleted messages in the conversion (which also may have some errors), you have to compact your folders in Thunderbird. Right click a folder and select “Compact This Folder”. This will completely remove deleted messages from the folders mbox file.

If you can’t find your mbox files, they are located in the Profile Folder, in the Mail and (if you use IMAP) ImapMail folders. Each mail folder (Inbox, Sent, etc.) is stored as two files:

  1. one file is with no extension (e.g. inbox), which is the mail file itself (in “mbox” format); this may be confusing because you’d probably expect a name like inbox.mbox. OK, so although “inbox” has no file extension associated to it, it is in a mbox format and that’s the file you’re looking for.
  2. the second file is the msf file, (e.g. INBOX.msf), which is the index (Mail Summary File) to the corresponding mail file.


  • I tried exporting a large number of emails, i.e. 4000 and it worked perfectly,
  • All the attachments in the mbox files are retained in the resulting files,
  • conversion of multiple mbox files at once is supported.

2. Importing files to Outlook

The second step you need to perform, to import messages from Thunderbird to Microsoft Outlook, is to import files to your MS Outlook, read this article I previously published: Import files to Microsoft Outlook.


Does this work for you? Leave us a comment.

Article Name
Import messages from Thunderbird to Microsoft Outlook
In this article learn how to import messages from Thunderbird to Microsoft Outlook.
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