Thunderbird to Outlook: a step-by-step migration guide

What moves, and what doesn’t

A Thunderbird profile holds three kinds of data: mail, contacts and calendars. They travel separately, because Thunderbird and Outlook store each of them differently.

• Mail is stored as mbox files, one per folder. This is the hard part, and the main subject of this guide.
• Contacts: in Thunderbird’s Address Book, each address book can be exported as a CSV or vCard file (look for an Export option on the address book). Outlook can import both.
• Calendars: a local Thunderbird calendar can be exported as an .ics file, which Outlook can import or open. Calendars that live on a server (Google, Microsoft 365, CalDAV) don’t need exporting; just connect that account in Outlook.

MoveMyInbox handles the mail only. Export contacts and calendars yourself before you uninstall Thunderbird; it takes a few minutes.

The process at a glance

1. Check whether your accounts are IMAP; if so, you may only need to add them to Outlook.
2. For IMAP accounts, make sure every message is downloaded to this computer.
3. Compact your folders.
4. Open the profile folder via Help → Troubleshooting Information → Profile Folder → Open Folder.
5. Close Thunderbird.
6. Convert the account folder from Mail or ImapMail to a PST, keeping one folder per mbox file.
7. Open or import the PST in Outlook.
8. Check folders and message counts before deleting anything.

First check: do you need to migrate mail at all?

Open Account Settings in Thunderbird and look at Server Settings for each account. The server type tells you what to do.

IMAP accounts: the mail is already on the server

With IMAP (Gmail, Outlook.com, iCloud, most work and hosting accounts), Thunderbird is only showing you a copy of what lives on the mail server. Add the same account to Outlook and it downloads the same folders. No conversion needed.

You only need a file-based migration for an IMAP account if you are closing that account, changing provider, or want an offline archive of it.

POP3 accounts and Local Folders: the mail exists only on your computer

POP3 downloads messages and usually removes them from the server. Anything in Local Folders, including mail you archived or moved there, is also stored only on your disk. For this mail, the files in your Thunderbird profile are the only copy, so they must be converted.

Prepare Thunderbird

Make sure IMAP mail is fully downloaded

If you are converting an IMAP account, Thunderbird may only hold message headers locally. To download complete messages:

1. Open Account Settings and select Synchronization & Storage under the IMAP account.
2. Tick Keep messages for this account on this computer. Under Advanced…, check that all the folders you want are selected, and remove any size limit on downloaded messages.
3. Wait for the download to finish. To force one folder, right-click it → Properties → Synchronization → Download Now.

Compact folders

When you delete a message, Thunderbird marks it as deleted inside the mbox file but leaves it there until the folder is compacted. Compact first, so those leftovers don’t reappear in Outlook: use File → Compact Folders, or right-click an account and choose Compact.

Find your Thunderbird profile folder

The reliable way, on every system: in Thunderbird, open Help → Troubleshooting Information (on Windows, press Alt if the menu bar is hidden). Next to Profile Folder, click Open Folder (Show in Finder on a Mac). The folder name looks like xxxxxxxx.default-release.

If you prefer to browse there yourself, these are the usual locations:

Windows

%APPDATA%\Thunderbird\Profiles\. Paste that into the File Explorer address bar or the Run box (Windows key + R).

macOS

~/Library/Thunderbird/Profiles/. The Library folder is hidden by default: in Finder choose Go → Go to Folder… and paste the path.

Linux

• Distribution package or Mozilla’s download: ~/.thunderbird/
• Snap package: ~/snap/thunderbird/common/.thunderbird/
• Flatpak: ~/.var/app/org.mozilla.Thunderbird/.thunderbird/. The Open Folder button may not work inside Flatpak, so browse there with hidden files shown (Ctrl+H in most file managers).

To find one account’s folder exactly, open its Server Settings: the Local Directory field at the bottom shows the full path.

What’s inside: Mail, ImapMail and mbox files

Inside the profile, mail lives in two directories, each with one subfolder per account:

• Mail: POP3 accounts and Local Folders.
• ImapMail: IMAP accounts, for example ImapMail/imap.gmail.com.

Within an account folder you’ll see a mix of files. Only some of them are mail:

How subfolders are stored

A folder’s subfolders sit in a directory with the same name plus .sbd. If you have a “Receipts” folder under Inbox, its mail is in Inbox.sbd/Receipts, with Inbox.sbd/Receipts.msf next to it. Deeper nesting just repeats the pattern, such as Inbox.sbd/Receipts.sbd/2025. Uploading the whole account folder keeps this structure together.

Convert the mailboxes to PST

Close Thunderbird first, so no file changes while you copy or upload it. Then pick a conversion method.

With MoveMyInbox

1. Start a new conversion and drag the account folder, for example Mail/Local Folders or ImapMail/imap.example.com, onto the upload area. Whole folders are supported, and .msf index files are skipped automatically.
2. Leave folders on One folder per mbox file, so Inbox, Sent and your own folders each become an Outlook folder of the same name, subfolders included.
3. Check the file list. Trash and Junk are converted too unless you remove them.
4. See the one-time price for the total size (details on the pricing page), pay with Stripe, and download the PST when the conversion finishes, typically within minutes.

Attachments, inline images, read and flagged status, dates and any character set are preserved. A message that can’t be parsed is never dropped; it is added with the original attached as an .eml file. It works from any browser, so you can do it from the Mac or Linux machine Thunderbird ran on. Uploads are deleted as soon as the PST is ready; see the security page for details.

Other options

If you would rather not upload mail, there are free and offline routes: copying folders into an IMAP account that Outlook also uses, or a desktop converter. Each has trade-offs in speed and folder handling, compared honestly in How to convert MBOX to PST: 4 methods compared.

Import the PST into Outlook

You have two choices for the PST, and both use classic Outlook for Windows:

• Open it as a separate data file (File → Open & Export → Open Outlook Data File). Your Thunderbird folders appear as their own tree in the folder pane. Good for an archive.
• Import it into your mailbox (File → Open & Export → Import/Export → Import from another program or file → Outlook Data File (.pst)). The folders are merged into your account and, with Microsoft 365 or Outlook.com, sync to your other devices.

Outlook for Mac can import a PST from its Import option. In the new Outlook for Windows, PST support is still limited and rolling out, so classic Outlook remains the reliable way to open or import one. Our PST import guide covers each version step by step.

Final checks before you uninstall Thunderbird

1. Compare folder names and message counts in Outlook with Thunderbird (both apps can show counts in folder properties).
2. Open a few old messages with attachments and make sure the files open.
3. Search for a name or a word with accents to confirm search and character sets work.
4. Import your exported contacts and calendar files, and set up your accounts and signatures in Outlook.
5. Keep the profile backup for a few months. Storage is cheap; lost mail isn’t.

Once everything checks out, you can remove Thunderbird. If you ever need to look inside the old mbox files again, how to open an MBOX file lists simple ways to do it.

Frequently asked questions