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How to search emails inside a large MBOX file
Practical ways to find one email among thousands: what to type, how to narrow results and what search cannot see.
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Thunderbird keeps your mail in ordinary files on disk. By default each folder, such as Inbox, Sent or Archives, is a single MBOX file. That means you can read old Thunderbird mail even when the program is not installed, the profile is broken or you are working from a backup drive.
Find the Thunderbird profile folder for MBox Viewer
If Thunderbird still runs, the easiest way is from inside the program: open Help → Troubleshooting Information, find Profile Folder and click Open Folder.
Otherwise, go to the default location for your system:
| System | Profile location |
|---|---|
| Windows | %APPDATA%\Thunderbird\Profiles\ |
| macOS | ~/Library/Thunderbird/Profiles/ |
| Linux | ~/.thunderbird/ |
Inside you will see one or more folders with random-looking names ending in something like .default-release. The most recently changed one is usually the active profile.
Which files hold the mail MBox Viewer reads
Inside the profile, mail is split by account type:
- Mail/Local Folders/ holds Local Folders and POP accounts.
- ImapMail/server name/ holds copies of IMAP accounts.
In each of these you will see pairs of files:
- Inbox, Sent, Drafts and so on, with no extension. These are the MBOX files.
- Inbox.msf, Sent.msf. These are summary indexes. They contain no messages and can be ignored.
- Folders ending in .sbd contain subfolders, with the same file pairs inside.
Open a Thunderbird folder in MBox Viewer
- Close Thunderbird if it is running, so the file is not being written to.
- Copy the file you want, for example
Inbox, to another folder such as your Desktop. - Start MBox Viewer from the free download and click File → Select folder….
- Choose the folder with your copy. MBox Viewer recognises archives by their content, so the extension-less file appears in the archive tree. Click it to load the messages.
Tip: on Windows, the AppData folder is hidden. Paste %APPDATA%\Thunderbird\Profiles into the File Explorer address bar to jump straight there.
Why MBox Viewer may show deleted messages
When you delete a message in Thunderbird, it is first only marked as deleted inside the MBOX file. It is physically removed when the folder is compacted. Until then, a viewer that reads the raw file can still show it.
This can be useful if you are looking for something deleted by mistake. If you want the file to match what Thunderbird shows, compact the folder first by right-clicking it and choosing Compact.
IMAP folders may be incomplete in MBox Viewer
For IMAP accounts, Thunderbird stores full message copies only when offline synchronisation is on. If it was off, the local file may contain headers only or a subset of messages. Check the account's Synchronisation & Storage settings if a file looks smaller than expected.
Maildir profiles and MBox Viewer
Thunderbird can also use Maildir storage, where each message is a separate file inside cur folders. If you see folders instead of single files, the profile uses Maildir, and an MBOX viewer is not the right tool. Thunderbird itself, or a Maildir-aware program, will read those.
