How to Back Up Thunderbird Emails

The short answer: Thunderbird gives you three ways to back up your mail — Tools → Export writes your whole profile to a zip (capped at about 2 GB), the ImportExportTools NG add-on saves folders as MBOX or individual .eml files, and copying the profile folder by hand covers profiles too big for the export tool. But before any of them, check one setting: if your IMAP account isn't keeping messages on your computer, there's nothing on disk to back up.

That last point is the one every guide skips, so let's start there.

Doesn't Thunderbird already store my email locally?

Partly — and "partly" is the problem.

Thunderbird is a desktop client, so people assume the mail is safely on their disk. For POP accounts, and for IMAP accounts with offline storage switched on, it is. But an IMAP account that isn't synchronizing only downloads message headers. You see subjects and senders because they're cached; the bodies are fetched from the server each time you open one. Mozilla's own documentation is blunt about the consequence: when messages aren't kept locally, there is no mbox file at all. Back up that profile and you've backed up a list of subject lines.

Even when everything is downloaded, a local Thunderbird store isn't a backup on its own:

• It's one copy on one disk — the same disk you're trying to insure against.
• IMAP is a mirror, not a vault. Delete a message on the server (or lose the account) and the next sync removes it locally too.
• The store is in Thunderbird's own layout, so "restoring one email" means getting Thunderbird running again first.

A backup is a separate copy that survives the original. Here's how to make one.

Step 0: make sure your mail is actually on your computer

Do this before any backup method below, per account:

1. Tools → Account Settings, pick your account, open Synchronization & Storage.
2. Under Message Synchronizing, tick Keep messages in all folders for this account on this computer.
3. Click Advanced… and confirm every folder you care about is checked — subfolders included.
4. Under Disk Space, choose to synchronize all messages locally regardless of age (some setups default to keeping only recent mail).
5. Leave Thunderbird open and let it download. A large mailbox takes a while the first time.

How to verify it worked: open your profile folder (Step 3 below shows where) and look inside Mail or ImapMail. Each downloaded folder appears as a file with no extension — Inbox, Sent — sitting next to a same-named .msf index file. If you see only the .msf and no extension-less twin, that folder's messages are still server-side.

Where Thunderbird keeps your email

Thunderbird's default storage format is MBOX: one plain-text file per folder, every message stacked inside it. There's also an optional maildir mode (one file per message) you can select for new accounts under the message store type setting — with a catch worth knowing: an account's storage format can't be changed after it's created. Switching means removing the account and setting it up again.

For backup purposes the default is fine, and MBOX is genuinely portable — any of these files opens in our free MBOX viewer without Thunderbird installed at all.

Method 1: Tools → Export (the built-in profile backup)

The simplest option, built into Thunderbird: it zips your entire profile — accounts, messages, address books, filters, settings — into one file.

1. Tools → Export.
2. Click Export, choose a destination and a name (thunderbird-backup-2026-08-09.zip).
3. To restore, use Tools → Import and pick the zip — typically on the new machine.

The 2 GB ceiling. Thunderbird's documentation states the export tool only supports backup zips up to about 2 GB. Mail archives cross that line easily, and this is the single most common reason the method fails people. If your profile is bigger, skip to Method 3.

A restore caveat: a profile exported from an old Thunderbird won't necessarily import into a much newer one — mismatches surface as "This folder does not contain a valid profile." Restore into the same version or newer, and don't leave a years-old zip as your only copy without testing it.

One security note nobody mentions: the profile contains your saved account passwords. Treat the zip as sensitive — keep it somewhere private, and set a Primary Password in Thunderbird if you haven't.

Good: one click, captures everything including settings, and restores cleanly to a new computer. Bad: the 2 GB cap, a Thunderbird-shaped format only Thunderbird reads, and it's a manual snapshot you have to remember to repeat.

Method 2: Export folders as MBOX or .eml (the portable one)

Method 1 backs up Thunderbird. This one backs up your email — in formats that outlive the app.

Install the ImportExportTools NG add-on (Thunderbird's add-on manager, or addons.thunderbird.net), then:

• Whole folder, one file: right-click a folder → ImportExportTools NG → Export folder for MBOX. It can include subfolders, so you can take an entire account in one pass.
• One file per message: right-click a folder → ImportExportTools NG → Export all messages in folder → EML format. You also get HTML, PDF, CSV, and plain text, but .eml is the one to choose — it's the original message, headers and attachments intact.
• A few messages: select them, right-click → Save selected messages → EML format.

Which to pick? MBOX is compact and imports back into a mail client easily. .eml is far better for actually finding and restoring one email later — every message is its own file your operating system can search, and no import step stands between you and reading it. You can convert either way afterwards with our free MBOX to EML and EML to MBOX converters, or read a single message in the EML viewer.

Good: open, portable formats that any client or tool can read; works at any size; you choose exactly which folders matter. Bad: still a manual snapshot, and it doesn't carry your settings, filters, or address book — pair it with Method 1 or 3 if you want those.

Method 3: Copy the profile folder (for large profiles)

The fallback when the export tool's 2 GB limit bites. It's a straight file copy — with one rule.

Close Thunderbird first. Copying an open profile can capture half-written mbox and index files, and a corrupt archive isn't a backup.

Then find the profile. Don't guess at paths — Thunderbird will tell you, and this is immune to OS differences and Flatpak/Snap installs:

Help → Troubleshooting Information → Profile Folder → Open Folder

That opens the folder directly. Quit Thunderbird, then copy the whole thing somewhere else. For reference, the defaults are:

Inside you'll find Mail (local and POP folders) and ImapMail (downloaded IMAP mail) — the extension-less MBOX files from Step 0 live in there, and you can lift any single folder out and read it in the MBOX viewer without restoring anything.

Good: no size limit, captures literally everything, no add-on required. Bad: manual, must be done with Thunderbird closed, and it contains your saved passwords — same caution as Method 1.

Method 4: Auto-forward new mail to your own cloud

Methods 1–3 all share the same weakness: they're snapshots. They're stale the moment they finish, and the copy usually sits on the same laptop you're insuring against.

The fix is to have new mail archived automatically as it arrives. You get a dedicated inbound address, mail is forwarded to it, and every message is written to storage you own — Google Drive, OneDrive, Dropbox, or S3 and S3-compatible buckets — as the original .eml, deduplicated by Message-ID so a retry never stores twice.

There are two ways to point mail at it, and the difference matters:

Thunderbird's own filter (works, with a real limitation). Thunderbird can forward automatically: Tools → Message Filters → New, set the condition (match all incoming mail, or just certain senders), and choose the action Forward Message To, with your archive address. It's genuinely useful — especially for selectively archiving mail from a few senders.

The limitation: filters only run while Thunderbird is open and checking mail. Close the laptop for the weekend and nothing is forwarded until you open it again. That's fine for a machine that's always on; it's not a backup you can rely on otherwise.

Provider-side forwarding (the one to prefer). Set the forward at your mail provider instead, and it runs on their servers whether or not your computer is on. Thunderbird is just a client — the account behind it is Gmail, Outlook, Yahoo, or iCloud, and each has its own path:

Good: continuous and automatic, files land in storage you control, and it survives your computer dying. Bad: forwarding starts today — it can't reach backwards into your history. Pair it with one export from Method 2 for the back catalogue.

Which method should you use?

The practical combination for most people: one .eml or MBOX export now for everything you already have, plus provider-side forwarding from today on so new mail archives itself. That's the same pairing we recommend for any mailbox — a snapshot for the past, automation for the future.

Moving Thunderbird to a new computer

Backup and migration overlap here, so briefly: install the same-or-newer Thunderbird version on the new machine, then either use Tools → Import with a zip from Method 1, or copy your old profile folder over the new one (both apps closed) using the paths above. Your accounts, mail, filters, and address book come with it. Passwords come too — which is exactly why the copy in transit deserves care.

FAQ

Where does Thunderbird store emails?

In your profile folder, as MBOX files — one plain-text file per mail folder, with no file extension, next to a matching .msf index. Local and POP mail sits in the Mail subfolder, downloaded IMAP mail in ImapMail. The fastest way to find your profile is Help → Troubleshooting Information → Profile Folder → Open Folder; the defaults are %APPDATA%\Thunderbird\Profiles\ on Windows, ~/Library/Thunderbird/Profiles/ on macOS, and ~/.thunderbird/ on Linux.

Does backing up my Thunderbird profile back up all my email?

Only the mail Thunderbird has actually downloaded. An IMAP account without offline storage enabled keeps message headers locally and fetches bodies from the server on demand — so the profile contains no mbox file for those folders and the backup captures almost nothing. Turn on Keep messages in all folders for this account on this computer in Account Settings → Synchronization & Storage, let it finish downloading, then back up.

Why does Thunderbird's export fail on a large profile?

Thunderbird's built-in export tool only supports backup zip files up to roughly 2 GB, and mail archives pass that quickly. For a larger profile, close Thunderbird and copy the profile folder manually, or export just your mail folders to MBOX or .eml with the ImportExportTools NG add-on.

How do I export Thunderbird emails as .eml files?

Install the ImportExportTools NG add-on, right-click the folder you want, and choose Export all messages in folder → EML format — you get one .eml file per message, headers and attachments intact. For a handful of messages, select them and use Save selected messages → EML format instead. Already have an MBOX export? Split it into individual messages with our free MBOX to EML converter.

Can Thunderbird forward emails automatically?

Yes — Tools → Message Filters, create a filter, and choose the Forward Message To action. The important caveat is that Thunderbird filters only run while Thunderbird is open and checking mail, so nothing is forwarded while your computer is off. For backup you can depend on, set the forward at your mail provider (Gmail, Outlook, iCloud) so it runs server-side instead.

Is MBOX or .eml better for a Thunderbird backup?

.eml for restoring, MBOX for moving. MBOX keeps a whole folder in one file that imports back into a mail client easily, but pulling a single message out of it means importing or splitting it first. One .eml per message is searchable by your operating system, opens in anything, and makes recovering a single email trivial — which is what you actually need from a backup at the moment you need it. For the format comparison in general, see MBOX vs .eml.