Short path

Quit Thunderbird. Open ~/Library/Thunderbird/Profiles/. Inside your profile, go to Mail/Local Folders/ (or Mail/ImapMail/<server>/). Copy the file named after the folder you want (e.g. Inbox), add a .mbox extension to the copy, and drag it into MBOX to CSV Data Extractor.

On this page
  1. Why convert Thunderbird to CSV
  2. Step 1 — Quit Thunderbird
  3. Step 2 — Open your profile folder
  4. Step 3 — Find the MBOX file
  5. Step 4 — Copy and add .mbox extension
  6. Step 5 — Convert
  7. Tips and gotchas
  8. FAQ

Why convert Thunderbird to CSV

Thunderbird's built-in search is fine for finding one message. It is not fine for:

  • Counting messages by sender or domain over a date range
  • Exporting a specific folder for e-discovery or legal hold
  • Migrating email data to a BI tool, database, or research corpus
  • Building a "who emailed me most in 2025" report

Once Thunderbird's MBOX is in CSV or Excel, every one of these becomes a 10-second task.

Step 1 — Quit Thunderbird

Thunderbird holds write locks on its mailbox files while running. Copying a live file gives you either a file-lock error or a half-written snapshot. Quit Thunderbird with Cmd+Q and wait a second or two for it to finish flushing to disk.

Step 2 — Open your Thunderbird profile folder

In Finder, press Shift+Cmd+G (Go to Folder) and paste:

~/Library/Thunderbird/Profiles/

You'll see one or more folders ending in .default or .default-release. Open the one for your active profile. If there's more than one and you're not sure which, Thunderbird → Help > Troubleshooting Information shows the path of the profile currently in use.

Step 3 — Find the MBOX file

Inside your profile, the mail storage lives under Mail/ with two common subfolders:

  • Mail/Local Folders/ — your Thunderbird-only folders (Drafts, Templates, Archives, anything you created locally).
  • Mail/ImapMail/<server>/ — local caches of IMAP folders (one subfolder per account server).

Each Thunderbird folder is stored as a single file with no extension, named exactly like the folder: Inbox, Sent, Archives. Next to each data file you'll see a companion .msf file — that's Thunderbird's summary index, not part of the MBOX format. Leave it alone; you don't need it for conversion.

Step 4 — Copy the file and add .mbox extension

Drag the target file (e.g. Inbox) to your Desktop. On the copy, press Return to rename it, and add .mbox:

Inbox   →   Inbox.mbox

macOS will ask whether you want to change the extension — say yes. The copy is what you'll convert; the original stays untouched.

Step 5 — Convert with MBOX to CSV Data Extractor

  1. Install MBOX to CSV Data Extractor from the Mac App Store (free for files under 50 MB).
  2. Open the app and drag your Inbox.mbox copy into the window.
  3. Pick CSV (or Excel / JSON / XML / TXT), choose a destination folder, and click Convert.

The app streams the file so even a multi-gigabyte Thunderbird folder converts without loading everything into memory.

Tips and gotchas

  • Multi-profile setups. If you switch between Thunderbird profiles, profile.ini in ~/Library/Thunderbird/ tells you which profile is the default. The "Troubleshooting Information" screen inside Thunderbird confirms which profile is currently active.
  • IMAP accounts with server-only storage. If you've set an IMAP account to not keep messages on disk, the MBOX files will be small or empty. Enable Keep messages for this account on this computer in the account's Synchronization & Storage settings, let Thunderbird sync, then quit and copy.
  • Compacting. Thunderbird's Compact Folders operation rewrites MBOX files to remove deleted-but-still-taking-space messages. If your folder feels oversized, run File > Compact Folders before quitting — the conversion will be faster.
  • Don't forget Sent, Archives, and subfolders. People usually think "my email" = Inbox, but Sent and Archives often hold the more interesting history.
  • Convert a copy, never the original. The app is read-only by design, but you'll sleep better.

FAQ

Where does Thunderbird store MBOX files on Mac?

Inside your profile folder at ~/Library/Thunderbird/Profiles/<profile-id>.default-release/Mail/. Local-only folders live under Mail/Local Folders/; IMAP folders under Mail/ImapMail/<server>/. Each folder is a single file with no extension, named after the folder.

Why do Thunderbird MBOX files have no extension?

Thunderbird uses the original Unix mbox convention — the filename is the mailbox name, no suffix. The .msf file next to each one is Thunderbird's summary index and is not part of the MBOX format. Rename your copy to add .mbox so other tools recognize it.

Can I convert an MBOX while Thunderbird is running?

Not reliably. Quit Thunderbird, copy the file, then convert the copy. Live files may be locked or mid-write.

What happens to IMAP folders?

Thunderbird caches IMAP folders locally under Mail/ImapMail/<server>/. The MBOX files contain whatever Thunderbird has downloaded. If your account is set to server-only, the files will be small — enable Keep messages on this computer and let it sync first.

Can I convert multiple Thunderbird folders at once?

Copy each target file to a work folder, rename with .mbox suffixes, and convert them one at a time. Each produces its own CSV — clean separation by source folder.

Does converting touch the original Thunderbird mailbox?

No. The app reads your file, writes output to a destination folder you choose, and never modifies the input. Still, always convert a copy rather than the original.

Related

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