# Why Your Mbox Import Fails in Thunderbird (and How to Fix It)

## What is going on here?

An mbox file is a kind of "box" that stores a large number of emails one after another inside a single text file. Thunderbird, macOS Mail and older mail programs use this format to keep entire mailboxes. It sounds simple, but in practice importing into Thunderbird often stalls: nothing seems to happen, your folder stays empty, or Thunderbird shows an error with no explanation.

In this article we walk through the five most common causes, with a concrete fix for each one. The good news: in almost every case your emails are not lost, you just need to convince Thunderbird to handle the file the right way.

:::tip title="Want a quick look before importing?"
If you simply want to check what is inside your mbox file before you start importing, drag it into **Mbox Viewer by Cloud Captains**. You instantly see the list of messages on the left and the open message on the right, just like in Gmail or Thunderbird. Everything runs locally on your own device, nothing goes to a server or the cloud. Handy for confirming you grabbed the right file. See https://mbox-viewer.online
:::

## Cause 1: the ImportExportTools NG add-on is missing or set up wrong

Thunderbird cannot import a standalone mbox file through a button on its own. For that you need the free **ImportExportTools NG** add-on. Most failed imports happen because this add-on is not installed, not enabled, or stopped working after a Thunderbird update.

:::howto title="Install and use ImportExportTools NG"
1. Open Thunderbird and click the menu in the top right (the three lines), then choose Add-ons and Themes.
2. Search for ImportExportTools NG in the search bar and click Add to Thunderbird on the correct result.
3. Restart Thunderbird so the add-on becomes active.
4. First create an empty folder to hold the messages: right click Local Folders and choose New Folder.
5. Right click that new folder, go to ImportExportTools NG and choose Import mbox file.
6. In the menu pick the option to import one or more mbox files directly, then select your file.
:::

:::warn title="Watch out after a major Thunderbird update"
After a major version update of Thunderbird, an add-on can be disabled temporarily because it is not yet compatible. Check in Add-ons and Themes whether ImportExportTools NG appears greyed out or disabled, and update it if needed.
:::

## Cause 2: the file is too large

Large mbox files (think of a full Gmail archive of several gigabytes) often stall during import. Thunderbird appears to freeze or stops halfway. This happens because the whole file has to be processed in one go.

:::howto title="Make an oversized file manageable"
1. First check the size of your file in your file manager (right click, then Properties or Get Info).
2. If the file is larger than roughly 4 gigabytes, split it into smaller parts. ImportExportTools NG has an option for this, or you can use a dedicated splitting tool.
3. Import the parts one by one into separate folders.
4. Give Thunderbird plenty of time during the import: with large files it can take minutes before anything shows up. Wait for the message that the import is finished before clicking on.
:::

:::tip title="Quickly confirm a large file is intact"
Before you start an import that could take hours, you can drag a hefty mbox file into Mbox Viewer first to see whether it opens and how many messages it holds. You set your own limit for how much is loaded into memory at once under Settings. That way you immediately know whether the file is usable.
:::

## Cause 3: the wrong or doubled file extension

Thunderbird and ImportExportTools NG expect a real mbox file. Sometimes a file has a confusing extension, for example because it is still packed up. A classic example is a file from Google Takeout that ends in .mbox.gz: that is a packed (compressed) file, not a ready to use mbox.

:::howto title="Check and fix the right extension"
1. Look at the full file name including the extension. If you see .gz, .zip or .tar at the end, you must unpack the file first.
2. Unpack a .gz file with a free extraction tool so you end up with a file without the .gz part.
3. Check that the unpacked file ends in .mbox. If the file is just called Inbox or Takeout without an extension, rename it to something like Inbox.mbox.
4. Try the import again with the unpacked file.
:::

:::tip title="Google Takeout is unpacked automatically in Mbox Viewer"
Got a .mbox.gz from Google Takeout and would rather not unpack it by hand? Drag it straight into Mbox Viewer, where a .mbox.gz is unpacked and opened automatically. That way you immediately see whether the contents are correct before importing into Thunderbird.
:::

## Cause 4: a corrupt or stuck index

For each folder Thunderbird keeps a separate index file (with the .msf extension) that records which messages it contains. If that index no longer matches the actual contents, an imported folder can look empty or damaged even though the messages are really in there.

:::howto title="Rebuild the index"
1. Right click the folder that looks empty or odd and choose Properties.
2. Go to the General Information tab and click Repair Folder.
3. Thunderbird now rebuilds the index. For a large folder this can take a while.
4. If that does not help, close Thunderbird completely, delete the matching .msf file in the profile folder (not the mbox file itself) and start Thunderbird again so the index is rebuilt automatically.
:::

:::danger title="Never delete the mbox file itself"
When cleaning up a corrupt index you only delete the file that ends in .msf. Never delete the file without an extension or with .mbox, because that is where your actual emails live. To be safe, always make a copy first.
:::

## Cause 5: missing read or write permissions

Sometimes the problem is not the file but the access permissions. If the mbox file sits in a location Thunderbird is not allowed to touch (for example a system folder, a read only drive or a network drive), the import fails without a clear reason.

:::howto title="Resolve permission problems"
1. Move the mbox file to a simple, personal location, for example your Documents folder or your Desktop.
2. Check that the file is not set to read only (right click, then Properties or Get Info, and clear the Read-only option).
3. If the file is on an external or network drive, copy it to your internal drive first and import from there.
4. Make sure you are signed in with your own user account and not trying to import from another user's folder.
:::

## Still no luck? View the file locally first

If the import really will not work, you want to know whether the problem is Thunderbird or the file itself. The fastest way to find out is to open the mbox file first in **Mbox Viewer by Cloud Captains**. You drag the file into the window and instantly see whether it is readable, how many messages it holds and whether attachments are present.

Everything runs fully locally and offline on your own computer. No files are uploaded, there is no server and there is no telemetry. Your emails stay yours. Besides reading, you can search with handy operators, label or star messages, and if you want, export to PDF, .eml or back to .mbox. That way you always have access to your archive, even if Thunderbird refuses the file for now.

:::faq
### Do I really need the ImportExportTools NG add-on to import an mbox into Thunderbird?
Yes, to directly import a standalone mbox file you need this free add-on. Thunderbird has no built-in button for it on its own. If you only want to view the file rather than import it, you can open it locally in Mbox Viewer without any add-on.

### My imported folder is empty, but the mbox file is large. Are my emails gone?
Usually not. This often points to a corrupt or stuck index. Right click the folder, choose Properties and repair the folder so the index is rebuilt. Your messages are still inside the mbox file.

### My file ends in .mbox.gz, why does that not work?
A .mbox.gz is a packed file, for example from Google Takeout. You have to unpack it into a plain mbox file before Thunderbird can import it. In Mbox Viewer a .mbox.gz is unpacked and opened automatically, so you can check the contents right away.

### Are my emails uploaded anywhere if I use Mbox Viewer?
No. Mbox Viewer works one hundred percent locally and offline. There is no server, no files are uploaded and there is no telemetry. Your data is stored only on your own device, and you can wipe it at any time through Settings and Clear database.

### Which file formats can I open as an alternative to importing?
Besides .mbox and .mbx you can also open .eml, .emlx, .msg, Maildir folders and .mbox.gz in Mbox Viewer. You drag the file or folder into the window and view the messages immediately, without having to import anything first.
:::