What is an mbox file?

An mbox file is a plain-text mailbox format that stores multiple email messages in a single file, separated by 'From ' lines. It is defined by RFC 4155 and is the standard archive format used by Gmail Takeout, Thunderbird, macOS Mail, Mutt, and most Unix-style mail systems.

How do I open an mbox file from Gmail Takeout?

Gmail Takeout exports each label as a single .mbox file. tomorrow-box opens that file directly — no import step, no conversion to EML or PST. Drop the file into the reader and browse, search, or thread your messages.

What is tomorrow-box?

tomorrow-box is a modern mbox viewer built to replace two-decade-old tools like Thunderbird, Mutt, and legacy mbox utilities. It reads Gmail Takeout archives, Unix mail spools, and forensic mbox files in software designed for today's hardware.

Is tomorrow-box available now?

tomorrow-box is in private beta during Spring 2026. Join the waitlist to receive an invite when seats open.

What mbox sources does tomorrow-box support?

tomorrow-box reads mbox files from Gmail Takeout, Thunderbird profiles, macOS Mail exports, Unix mail spools (/var/mail), and forensic archives produced by e-discovery tooling.

How to open a Thunderbird mbox file on a modern Mac or PC

Thunderbird stores each local folder as an mbox file (no extension) inside your profile directory. Close Thunderbird, copy the file out of the profile, rename it to .mbox, and open it in any mbox reader.

Where Thunderbird keeps the files

OS	Path
Windows	`%APPDATA%\Thunderbird\Profiles\<random>.default\Mail\Local Folders\`
macOS	`~/Library/Thunderbird/Profiles/<random>.default/Mail/Local Folders/`
Linux	`~/.thunderbird/<random>.default/Mail/Local Folders/`

Inside that folder, a file called Inbox (no extension) is your local inbox mbox. Inbox.msf is the summary index — ignore it; you only need Inbox. IMAP accounts live under Mail/ImapMail/<server>/ instead of Local Folders/.

Why open them outside Thunderbird

Thunderbird won’t start (profile corruption is common on version upgrades)
You’ve moved to Apple Mail or Outlook, and Thunderbird is no longer installed
You want fast search across the archive without loading Thunderbird’s UI
Legal or discovery request for a specific folder

Step-by-step: extracting a Thunderbird mbox

Close Thunderbird. The app locks profile files while running; copying a locked file produces corruption.
Navigate to the profile folder using the table above.
Copy the mbox file (the one with no extension) to a working directory outside the profile.
Rename it with a .mbox extension for clarity: Inbox → Inbox.mbox.
Open it in a reader:
- Mutt (Unix/macOS): mutt -f Inbox.mbox
- Python: import mailbox; box = mailbox.mbox("Inbox.mbox")
- tomorrow-box: drag the file in
- Another Thunderbird profile: drop into its Mail/Local Folders/, restart Thunderbird

Why not just fix Thunderbird?

Thunderbird’s UI has barely changed in two decades. Search is single-threaded and slows to a crawl on archives over a few gigabytes. Threading is primitive, keyboard shortcuts are inconsistent across platforms, and the profile locks while running. For reading an existing archive, a purpose-built mbox viewer is faster and doesn’t tie up a whole application.

Mbox variants

Thunderbird writes mboxrd (with >From escaping in message bodies). Any standards-compliant mbox reader handles this automatically — no manual conversion needed.

Common pitfalls

Copying while Thunderbird runs → corrupt file. Always quit first.
Grabbing the .msf file — that’s Thunderbird’s internal index, not your mail. You want the extensionless file with the same name.
Forgetting sub-folders. Nested folders in Thunderbird live under a sibling .sbd directory. Example: folder Archive stored on disk as Archive (the mbox) + Archive.sbd/ (a directory with child mboxes).

When the file is huge

Archives over ~10 GB become painful in Thunderbird. If that describes your Inbox or All Mail folder, extract the mbox file and open it in a reader built for large archives. A modern mbox viewer like tomorrow-box opens multi-gigabyte files without re-indexing from scratch each session.