How to export Thunderbird mail to MBOX files

Thunderbird already speaks MBOX

MBOX is one of the oldest mail formats around: a single text file with every message in a folder stored one after the other. Thunderbird’s default storage (called “File per folder” in its settings) uses exactly this. Your Inbox is a file called Inbox, with no extension, sitting in your profile folder.

That means there are three realistic ways to end up with MBOX files you can keep or convert:

Whichever you choose, do the preparation in the next section first. Most “my export is missing mail” stories trace back to skipping it. For background on how MBOX compares with EML, PST and the rest, see email file formats explained.

Before you export: download, compact, check storage

Make sure IMAP mail is actually on your computer

POP3 accounts and Local Folders keep complete messages on your disk. IMAP accounts (Gmail, Outlook.com, iCloud, most work mail) are different: depending on your settings, Thunderbird may only have downloaded message headers, or only some folders. The MBOX file for a folder that was never synced can be tiny, or missing entirely, even though the folder looks full in Thunderbird.

Under Account Settings → Synchronization & Storage, tick Keep messages for this account on this computer, check the folder selection under Advanced…, and remove any “don’t download messages larger than” limit. Then give Thunderbird time to finish. Our Thunderbird guide has the step-by-step version.

Compact your folders

Deleting a message in Thunderbird only hides it inside the MBOX file until the folder is compacted. If you copy the raw file, those “deleted” messages come along and can reappear in whatever opens it next. Run File → Compact Folders (or right-click the account and choose Compact) before you copy anything.

Check whether the account uses maildir

Thunderbird can also store mail as “File per message” (maildir), with each email as its own file. Open Account Settings → Server Settings for the account and look at Message Store Type in the Message Storage area. If it says file per message, there are no MBOX files to copy, and the add-on method below is the way to go.

Method 1: Copy the MBOX files from your profile

Open Help → Troubleshooting Information (press Alt on Windows if the menu bar is hidden, or use the ☰ menu → Help) and click Open Folder next to Profile Folder. On a Mac the button says Show in Finder. Then close Thunderbird, so nothing writes to the files while you copy them.

Inside the profile, look in two folders:

• Mail: POP3 accounts and Local Folders.
• ImapMail: one subfolder per IMAP account, such as imap.gmail.com.

In each account folder, the files without an extension (Inbox, Sent, Archives) are your MBOX mailboxes. Each has a matching .msf file, which is only an index Thunderbird rebuilds on its own. Subfolders live in a directory with the same name plus .sbd, so the folder “Clients” inside your Inbox is the file Inbox.sbd/Clients.

The simplest backup is to copy the whole account folder, .msf files and all, to another drive. You keep the folder structure, and you don’t have to pick files one by one. If you only need a few folders, copy just those extension-less files (and their .sbd directories if they have subfolders).

If you’re unsure where profiles live on Windows, macOS or the various Linux packages, the Thunderbird to Outlook guide lists every location.

Method 2: Export folders with ImportExportTools NG

ImportExportTools NG is a free, long-running add-on now maintained under the Thunderbird project on GitHub. Install it from Add-ons and Themes (☰ menu or Tools menu), search for its name, and restart Thunderbird if asked. It adds an ImportExportTools NG submenu to the folder right-click menu and to the Tools menu.

To export folders as MBOX:

1. Right-click the folder you want in the folder pane.
2. Choose ImportExportTools NG, then the option to export the folder. There are variants that include subfolders, either keeping their structure or not; the exact wording differs a little between versions.
3. Pick a destination directory. The add-on writes one MBOX file per folder there.

The same submenu can also export messages as EML, HTML, PDF, plain text or CSV, and the Tools menu entry offers full profile backups, including an optional automatic backup when Thunderbird closes. For MBOX, stick with the folder export options.

Because the add-on writes new MBOX files from what Thunderbird shows you, it works for maildir accounts too, which is its main advantage over copying files. It still can’t export messages that were never downloaded, so the IMAP preparation above applies just as much.

Method 3: Thunderbird’s built-in Export (and why it’s not MBOX)

Recent Thunderbird versions have an Export tool under Tools → Export (☰ menu → Tools → Export if you don’t show the menu bar). It packs your entire profile (accounts, messages, address books and settings) into a single zip file, meant to be restored with Thunderbird’s Import tool on another computer.

Two things to know before you rely on it:

• It’s a profile backup, not an MBOX export. The MBOX files are inside the zip, in the same Mail and ImapMail layout as Method 1. To use them elsewhere, you unzip and dig them out anyway.
• Mozilla documents a 2 GB limit. Its support article says the Export tool only supports backup zips up to 2 GB and recommends copying the profile folder manually for bigger profiles. Users report mixed results above that size, so don’t count on it for a large mailbox.

Use it when you’re moving Thunderbird itself to a new machine and your profile is small. For getting mail out of Thunderbird, Methods 1 and 2 are more direct.

Check the export before you trust it

A backup you haven’t checked is a hope, not a backup. A few minutes of checking catches nearly every problem:

• Compare message counts. Note how many messages Thunderbird shows for your biggest folders (the total appears in the status bar when you select a folder). Then count the messages in the exported copy, for example by importing it into a test folder or opening it in a converter that reports totals.
• Look for suspiciously small files. An IMAP folder with thousands of messages but an MBOX file of a few kilobytes was never downloaded. Go back and sync it.
• Open one file elsewhere. Pick a folder and open the exported copy in another app or viewer. If an old message with an attachment opens correctly, the format is fine.
• Spot-check the oldest mail. Search the copy for something from years ago. Old mail is what’s most likely to have been left on the server.

What to do with your MBOX files

MBOX files are a good long-term format: they’re plain text, documented and readable by many apps. From here you can:

• Keep them as an archive. Many mail apps and viewers can open them later.
• Import them into another Thunderbird with ImportExportTools NG, or by copying them into a profile’s Local Folders directory while Thunderbird is closed.
• Convert them for Outlook. Outlook can’t open MBOX, so the files have to become a PST first. With MoveMyInbox you can drop the whole account folder (or select individual files) into the browser; index and metadata files are skipped, and each mailbox becomes an Outlook folder of the same name. Zip files aren’t accepted, so unzip a profile export first. Then follow our guide to importing the PST, or the new Outlook version.

Frequently asked questions