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Thunderbird MBOX files: find, export, import and open them

Drop an .eml, .msg, or .mbox file here, or use Open a file above.

Thunderbird is one of the most popular desktop mail clients, and under the hood it stores your mail as MBOX files, one per folder. That makes it easy to back up, move and archive your mail, once you know where the files are and how they're named. This guide covers finding the files on each operating system, exporting and importing folders, and opening the files without Thunderbird.

How Thunderbird stores mail

Every Thunderbird profile has a folder on disk. Inside it:

Messages you delete in Thunderbird stay in the MBOX file, hidden, until the folder is compacted. Choose File → Compact Folders before copying files if you don't want deleted mail included.

Where is the Thunderbird profile?

The reliable way on any system is Help → Troubleshooting Information → Profile Folder → Open Folder (press Alt if the menu bar is hidden). The usual default locations are:

System Profile location
Windows %APPDATA%\Thunderbird\Profiles\ then the folder ending in .default-release
macOS ~/Library/Thunderbird/Profiles/ (in Finder use Go → Go to Folder)
Linux ~/.thunderbird/ (Snap installs: ~/snap/thunderbird/common/.thunderbird/; Flatpak: ~/.var/app/org.mozilla.Thunderbird/.thunderbird/)

To see where a particular account keeps its mail, open Account Settings, choose the account's Server Settings, and look at Local Directory at the bottom.

Step by step: find and copy a folder's MBOX file

Exporting folders with ImportExportTools NG

The free ImportExportTools NG add-on gives you menu commands for exporting and importing:

  1. Open Tools → Add-ons and Themes, search for ImportExportTools NG and add it.
  2. Right-click the folder you want to export.
  3. Choose ImportExportTools NG → Export folder to save it as an MBOX file, or pick another option to export messages as EML, HTML, PDF or plain text.

It's the easiest way to get a properly named .mbox file with subfolders, and works for IMAP folders as long as their messages have been downloaded.

Importing an MBOX file into Thunderbird

With ImportExportTools NG

  1. Right-click Local Folders (or the folder that should hold the import).
  2. Choose ImportExportTools NG → Import mbox file.
  3. Choose to import as a new folder, then select the file.

This works for MBOX files from anywhere, including a Google Takeout MBOX. Gmail labels won't be recreated; all messages land in one folder.

Without an add-on

  1. Open the profile folder as above and quit Thunderbird.
  2. Copy the MBOX file into Mail/Local Folders/.
  3. Rename it to remove any extension (for example Takeout instead of Takeout.mbox).
  4. Start Thunderbird. The folder appears under Local Folders and Thunderbird builds a new index.

Moving a whole profile

To move all your Thunderbird mail to a new computer, use Thunderbird's own File → Export (or Tools → Export) to create a profile backup ZIP, and Tools → Import on the new computer to restore it. This keeps accounts and settings as well as mail.

Opening Thunderbird MBOX files without Thunderbird

Sometimes Thunderbird isn't an option: the old computer died and you only have the drive, you're on a work machine where you can't install software, or you need to give a colleague a folder of email. The files are plain MBOX, so any MBOX reader can open them.

The quickest is the MBOX viewer in your browser. Drop the extensionless file, such as Inbox, and it opens as a searchable inbox. Don't drop the .msf file; if you do, InboxVue explains that it's an index and asks for the file with the same name and no extension. If your account used Maildir storage, each message is a separate file, which you can open with the EML viewer.

To turn a folder into documents, use MBOX to PDF. For every other way to open MBOX files, see how to open an MBOX file.

Troubleshooting

A folder appears empty after copying. Delete the folder's .msf file (not the mail file) with Thunderbird closed, then restart so it rebuilds the index.

The file is much bigger than the folder looks. It still contains deleted messages. Compact the folder in Thunderbird and copy it again.

My IMAP folder file has almost nothing in it. Messages weren't stored offline. In Account Settings → Synchronization & Storage, turn on Keep messages for this account on this computer, wait for the download to finish, then copy the file.

I can't find the profile on a Mac. The Library folder is hidden. In Finder, choose Go → Go to Folder and type ~/Library/Thunderbird/Profiles/.

Subfolders are missing. They're stored in the .sbd directory next to the parent folder's file. Copy that directory too.

Thunderbird says the folder is in use. Quit Thunderbird completely before copying or replacing files, so it doesn't change them mid-copy.

Summary

Each Thunderbird folder is an extensionless MBOX file in your profile, with a .msf index beside it and subfolders in .sbd directories. Use Troubleshooting Information to find the profile, ImportExportTools NG or a manual copy to move folders in and out, and a browser MBOX viewer when you need to read the files without Thunderbird.

FAQ

Does Thunderbird use the MBOX format?
Yes, by default. Each folder is stored as one MBOX file in your profile. Thunderbird also supports a Maildir-style option (one file per message), set per account under Server Settings, Message Storage, but MBOX remains the default.
What are the .msf files?
They're Thunderbird's summary indexes for each folder, used to display the message list quickly. They don't contain your emails. Thunderbird rebuilds them if they're deleted, and you don't need them to open an MBOX elsewhere.
Can I import an MBOX file into Thunderbird without an add-on?
Yes. Quit Thunderbird, copy the MBOX file into your profile's Mail/Local Folders directory with no file extension, and restart. The file appears as a folder under Local Folders.
Why is my IMAP folder's MBOX file small or empty?
For IMAP accounts Thunderbird only stores full messages locally when synchronisation is turned on for that folder. Otherwise the local file may hold only headers or nothing at all. Turn on Keep messages for this account on this computer under Synchronization & Storage, let it download, then copy the file.
How do I open a Thunderbird MBOX file on another computer without Thunderbird?
Copy the extensionless folder file and open it in a browser-based MBOX viewer. InboxVue reads Thunderbird files directly, including large ones, without installing anything.