Migrate Thunderbird to Outlook: MBOX Export and IMAP Upload

Thunderbird stores your mail as local MBOX files. Outlook expects PST, OST, or whatever IMAP gives it. That mismatch is the entire reason a Thunderbird-to-Outlook migration is harder than it looks. You cannot just drag the folders across, and the Outlook desktop import wizard refuses MBOX entirely. The reliable path is to treat the Thunderbird profile as your source of truth, then push every message into Outlook over IMAP so the server, not the desktop client, holds the migrated mail.

What you are actually moving#

A Thunderbird profile contains more than mail. Address books live in abook.sqlite, calendars in calendar-data/, filters in msgFilterRules.dat, and account credentials in encrypted form. For this migration you only need the message stores. Anything else either has a separate path (CSV export for contacts, ICS export for calendars) or does not survive the jump at all (local filters become Outlook rules, but they have to be rebuilt by hand).

The message stores themselves are MBOX files with no extension. A folder called Receipts is a single file named Receipts, and a matching Receipts.msf index sits next to it. Ignore the .msf files. They are summary indexes Thunderbird rebuilds on demand.

Where Thunderbird hides the profile#

On Windows, profiles sit under %APPDATA%\Thunderbird\Profiles\<random-string>.default-release. On macOS it is ~/Library/Thunderbird/Profiles/<random-string>.default-release. On Linux it is ~/.thunderbird/<random-string>.default-release. Inside the profile, look for Mail/ (POP accounts and Local Folders) and ImapMail/ (cached IMAP accounts). The folder names under each are the source server hostnames.

The fastest way to find the profile is to open Thunderbird, go to Help, then Troubleshooting Information, and click Open Folder next to Profile Folder. Close Thunderbird before touching any of those files. Live profiles get corrupted if a migration tool reads MBOX while Thunderbird is rewriting it.

Pick your migration path#

There are three realistic routes and one wishful one.

The wishful one is "Outlook will import MBOX." It will not. Outlook 2016, 2019, 2021, and the Microsoft 365 desktop client all reject MBOX. Stop looking for the menu item.

The three real options are:

Path A: IMAP-to-IMAP. If your Thunderbird account is already IMAP-backed (most modern mail is), you do not need MBOX at all. You point a migration tool at the original IMAP server and at Outlook, and the tool moves messages server to server. Thunderbird is just a viewer in this case.

Path B: Local MBOX to IMAP upload. If the mail you care about is in Local Folders or in a POP account, the messages live only on your machine. You need a tool that can read MBOX and APPEND messages over IMAP to Outlook. This is what most Thunderbird migrations actually require.

Path C: MBOX to PST conversion, then Outlook import. Convert each MBOX to a PST file using a converter, then use Outlook's File > Open & Export > Open Outlook Data File. This works but bloats disk usage and leaves you with a desktop-only copy. Skip it unless you are migrating to a non-IMAP Outlook profile.

Path B is the right answer for most readers, so the rest of this guide assumes it.

Source-side preparation#

Before you touch anything destination-side, get the Thunderbird profile into a known state.

Compact every folder#

Thunderbird's MBOX files grow forever until you compact them. A "deleted" message is just marked as deleted inside the MBOX, taking up space and confusing migration tools. Right-click each account in the folder pane and choose Compact Folders. For a heavy mailbox this can take 20 to 40 minutes and roughly halves the on-disk size.

Repair index files#

If a folder shows the wrong message count or refuses to open, right-click it and choose Properties, then Repair Folder. This rebuilds the .msf index. Do this on any folder you do not fully trust before you stage the migration.

Make a profile snapshot#

Close Thunderbird. Copy the entire profile folder somewhere safe. The first time something goes sideways, you will want to point a fresh Thunderbird at the snapshot to confirm what was in the source. This is non-negotiable for any migration above a single mailbox.

Destination-side preparation#

Outlook has two flavours that matter here. Outlook.com is the consumer service at imap-mail.outlook.com. Microsoft 365 (formerly Office 365) is the corporate service at outlook.office365.com. The IMAP details and quota differ, and OAuth2 token scopes differ.

For Outlook.com:

• IMAP host: imap-mail.outlook.com, port 993, SSL
• SMTP host: smtp-mail.outlook.com, port 587, STARTTLS
• Authentication: OAuth2 preferred. App passwords required when two-step verification is enabled and OAuth2 is not supported by your tool.

For Microsoft 365 mailboxes you happen to be calling "Outlook" internally:

• IMAP host: outlook.office365.com, port 993, SSL
• SMTP host: smtp.office365.com, port 587, STARTTLS
• Authentication: OAuth2 with the IMAP.AccessAsUser.All scope, or modern auth with app password when conditional access allows it.

If you are unsure which one you have, sign in at outlook.com. If you land on the consumer Outlook web app, you are on Outlook.com. If you land on Outlook on the web for business, you are on Microsoft 365 and you should read our guide to migrating Thunderbird straight to Office 365 instead, because the auth model is different.

Enable IMAP on Outlook.com#

In Outlook.com settings, search for IMAP and confirm it is on. For some legacy accounts it ships off by default. While you are there, raise the message size limit if there is a setting available — Outlook.com rejects messages over roughly 150 MB outright.

The migration itself#

Here is the actual procedure once both sides are ready.

1. Stage Thunderbird as a readable source

   Either point your migration tool at the live IMAP server behind the Thunderbird account, or point it at the MBOX files in the profile. A desktop-first migration tool like Mailbox Taxi reads MBOX directly so you do not need to spin up a local IMAP server.

2. Connect Outlook with the right auth

   Use OAuth2 wherever possible. App passwords are a fallback when OAuth is unavailable. Test the connection with a single throwaway message before kicking off the real run.

3. Map folders sensibly

   Inbox, Sent, Drafts, Trash, and Junk should map to Outlook's Inbox, Sent Items, Drafts, Deleted Items, and Junk Email. Nested folders move under a parent of your choice. Local Folders content usually goes under an "Archive from Thunderbird" parent so you can find it later.

4. Run a 200-message pilot

   Pick one mid-sized folder and migrate only it. Verify timestamps, read flags, attachments, and special characters in folder names. If anything is wrong, fix it now, not after 80 GB has moved.

5. Run the full migration in waves

   Move oldest folders first while the user is still on Thunderbird. The day before cutover, do an incremental pass for new mail. On cutover day, reconfigure the Outlook client and disable the Thunderbird account.

6. Verify counts and spot-check messages

   Compare folder counts in Thunderbird and Outlook. Open ten random messages in each major folder. Check that calendar invitations, encrypted messages, and signed messages render.

Errors you will probably see#

Real migrations do not run clean end to end. These are the ones that show up most often.

AUTHENTICATIONFAILED from Outlook. Usually means MFA is on and you supplied a regular password. Generate an app password or switch to OAuth2.

Too many simultaneous connections from Outlook.com. The consumer service caps you at a small number of concurrent IMAP connections. Drop your tool's parallelism to two and try again.

Folder UTF-7 conversion error. A folder name contains a character your tool is not encoding to IMAP UTF-7 properly. Rename the source folder to ASCII as a workaround.

Message too large for destination. Outlook.com rejects very large messages. Either skip those, or split attachments, or move them to a Microsoft 365 mailbox if one is available.

STARTTLS handshake failed against SMTP. Not your problem for migration since you only need IMAP, but worth noting if you reconfigure clients afterwards.

Why the MBOX path matters#

For a clean break from Thunderbird, the destination needs every message Thunderbird had locally. If you migrate via IMAP-to-IMAP, you miss any Local Folders content because it never touched a server. The MBOX route is the only way to be sure. The trade-off is that MBOX uploaders run slower than server-to-server tools because each message has to be parsed, APPENDed, and acknowledged individually.

Plan on 50–80 MB per minute realistic throughput against Outlook.com, and 80–120 MB per minute against Microsoft 365. A 10 GB mailbox is therefore a 2 to 4 hour job, not the 20 minutes the file copy time would suggest.

Things the docs do not mention#

A few details that will save you a support ticket later.

Outlook.com's Sent folder is named Sent in IMAP, not Sent Items. The Sent Items name is the display name in the desktop client. Configure your folder mapping for the IMAP name. Microsoft 365 is the opposite — the IMAP name is Sent Items.

Thunderbird's Junk folder usually does not contain anything worth migrating. Skip it unless your user has been training it for years and explicitly asks for it.

Message-IDs are preserved by IMAP APPEND, but the X-headers Thunderbird adds for its own filters are stripped by some uploaders. If a user relies on Thunderbird-era filters, document them before the move so they can be recreated as Outlook rules.

The MBOX format article goes deeper into how messages are framed in the file and why some uploaders mis-parse them. If your pilot shows truncated messages, read that next. The broader PST, MBOX, and EML migration guide covers cross-format conversions if you end up needing Path C.

After the move#

Switch the user's desktop client to Outlook, point it at the IMAP or Exchange Online endpoint, and confirm that all migrated folders show up. Leave Thunderbird installed but configured to a local-only profile for two weeks. If a folder is missing or a message looks wrong, you can still answer the question by going back to the original Thunderbird profile.

Rebuild filters as Outlook rules in the desktop client (File > Manage Rules & Alerts) or in Outlook on the web. Address books need to be exported from Thunderbird as LDIF or CSV, then imported into Outlook contacts. Calendars are an ICS export plus an Outlook calendar import, and they rarely come across with all colour categories intact.

If you are eyeing the same source for a different destination, the Thunderbird to Gmail path uses an almost identical MBOX-extract pattern but a different upload tool, and the migrate MBOX to Outlook post covers the same upload step in isolation if you already have MBOX from a source other than Thunderbird.