Import Thunderbird mail into classic Outlook

This guide uses classic Outlook, the Windows desktop app with a File tab. The MBOX converter requires classic Outlook; the File-menu export steps below also describe that app. An MBOX file stores many email messages, while a PST is the data file that classic Outlook can open.

Do you need to convert anything?

If the required mail is already visible in your provider's webmail, add that same account to Outlook and let it synchronize. You do not need to convert Thunderbird's local cache just to read those server messages in a different app.

Check Local Folders, old POP accounts, and folders you previously archived to disk separately. Their mail may exist only in Thunderbird. Signing in to Outlook cannot retrieve those local-only messages from the server. Use MBOX conversion or the upload route for that portion of the mailbox, and keep server mail out of the conversion batch if you do not want a second copy.

Check Thunderbird's message-store type first

Before choosing a method, check how Thunderbird stores your mail. Its usual File per folder (mbox) setting puts all messages from a folder such as Inbox into one mailbox file, often without a filename extension. The alternative File per message (maildir) setting creates a separate file for every message.

Open Account Settings, select the source account's Server Settings, and read Message Store Type. Check Local Folders separately if that is where your mail is stored.

• If it says File per folder (mbox), you can convert the mailbox files directly to PST.
• If it says File per message (maildir), do not select that directory in an MBOX converter. Use the IMAP or EML route described in this guide.

Before moving anything, open Help → Troubleshooting Information → Profile Folder → Open Folder. Close Thunderbird, then copy the complete profile folder to separate storage. Also copy any Local Directory stored outside the profile. Our Local Folders backup guide shows how to find those paths. Keep these copies unchanged in case you need to start again.

Choose one of the three routes

For most complete, local Thunderbird mailboxes, direct MBOX-to-PST conversion is the shortest route. IMAP works for mail already on the server or for local messages you copy into an authorized account. EML is practical for a small selection.

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Convert local MBOX mail to PST

Use this route after confirming that the source uses File per folder (mbox). The converter reads those mailbox files and creates a PST that classic Outlook can open, avoiding an upload through another mail account.

Step 1. Make the source local and complete

If the source is an IMAP account, make sure Thunderbird has downloaded the messages you need. One practical option is to copy the required folders into Thunderbird's Local Folders, then open a few messages and attachments before closing Thunderbird.

Step 2. Locate and copy the mbox tree

Open Account Settings → Local Folders and note the Local Directory path.

Close Thunderbird before copying the files. In a normal Thunderbird MBOX store:

• the mailbox file has the folder name and usually no extension;
• the matching .msf file is an index, not the mailbox itself;
• the matching .sbd directory contains mailboxes for nested folders.

Copy each mailbox file together with its .sbd directory, preserving their relative layout. Work from this copy and leave the profile backup untouched.

Step 3. Convert the copied files

Install the MBOX to PST converter. Our installer includes Convert MBOX to PST as part of ReliefJet Essentials. Open the utility from the standalone ReliefJet Essentials launcher or from the ReliefJet tab in classic Outlook.

1. Select the working folder that contains the copied mailbox files.
2. Enable Include subfolders to process nested source folders.
3. Choose a path and filename for the new PST.
4. If the mailbox is large, set a practical maximum PST size.
5. Click Run and keep the conversion report.

Skip files greater than refers to an entire source MBOX file and uses KB. It does not filter individual messages or attachments. Leave it at 0 if you do not want to exclude any source file.

Step 4. Open and verify the PST

In classic Outlook, choose File → Open & Export → Open Outlook Data File. Check the folder tree, an old and a recent message, and several attachments. Keep the Thunderbird backup until the PST passes these checks and you have copied it to separate storage.

Copy local mail through an IMAP account

IMAP is the standard method mail apps use to keep messages synchronized with a mail server. In this route, Thunderbird uploads copied messages to an IMAP account and Outlook downloads them from the same account. Because the account stays synchronized, copy messages rather than moving or deleting them.

Step 1. Add the IMAP account to Thunderbird

Open Thunderbird's account setup, enter the account address, and choose IMAP. Complete the provider's browser sign-in when it appears.

For personal Gmail accounts, IMAP has been permanently enabled since January 2025, so current settings no longer show an Enable IMAP switch. Choose Sign in with Google. An app password is only a fallback for an older client that cannot use Google's sign-in window.

Step 2. Copy mail to the IMAP account

Create a clearly named folder under the IMAP account. In each Thunderbird source folder, select the messages, choose Copy To, and select that destination.

Copy manageable batches and wait for Thunderbird to finish uploading each one. Check the messages in the provider's webmail before continuing. The provider's storage quota and attachment-size limits still apply.

Step 3. Add the same account to classic Outlook

In classic Outlook, choose File → Add Account or File → Account Settings → Account Settings → New. Select the provider account and complete its browser sign-in.

The screenshot below shows an older account dialog. For Google accounts, use the current Google sign-in window rather than entering a password into the fields shown here.

Use an app password only when the provider requires two-step verification and your Outlook version cannot complete the modern sign-in flow.

Step 4. Wait for Outlook to finish

Choose Send/Receive All Folders and leave Outlook open until it has downloaded the copied folders.

Open an older message and an attachment while the computer is offline. If both open, the content is stored locally. To make a PST, choose File → Open & Export → Import/Export → Export to a file → Outlook Data File (.pst).

Move selected mail as EML

An EML file contains one email message. This route does not need an intermediate IMAP account, but it loses the automatic folder structure and is awkward for a large mailbox. Outlook's EML drag-and-drop support also varies by version, so try a few messages first.

Step 1. Export messages from Thunderbird

Open a Thunderbird folder, select the messages, and use Save As or drag them to a folder on disk. You should see one .eml file for each message.

Step 2. Move the EML files into Outlook

Create the matching destination folder in classic Outlook, then drag in a small batch of EML files. Verify that new items appear inside the destination mail folder, rather than just opening an EML window or attaching files to a draft. Then open several imported items and check their sender, date, body, and attachments. If dragging does not create mailbox items, stop and use another route.

Repeat this for every Thunderbird folder you want to keep. The EML files do not carry the Thunderbird folder tree, so you must recreate that tree in Outlook. Smaller batches are easier to retry if either app stops responding.

Why are Thunderbird folders or messages missing in Outlook?

Start with one missing message whose sender, date, and subject you know. Find it in Thunderbird, identify the folder that actually contains it, and trace that folder through the chosen route. A search result or unified Inbox can show mail from several accounts; it does not identify one source mailbox file.

What transfers besides email?

MBOX conversion moves messages and their attachments, not Thunderbird's complete application profile. Account passwords, add-ons, filters, address books, and calendar subscriptions need separate setup. Keep the profile backup even after Outlook can read the PST; it contains settings that the PST cannot replace.

Move contacts separately

These routes move email only. Export the Thunderbird address book as CSV. In classic Outlook, use File → Open & Export → Import/Export → Import from another program or file → Comma Separated Values, select a Contacts destination, and map the fields. Try a few contacts first, especially names with accents and entries with several email addresses. A multi-contact vCard file needs a different import route; it is not interchangeable with CSV in this wizard.

If you use Thunderbird on a Mac or Linux computer, copy the confirmed MBOX source to a Windows computer that meets the converter requirements. Installing new Outlook alone does not supply the classic Outlook components this converter needs.

Can you use new Outlook instead?

For server mail, check whether new Outlook can connect to the original account. For MBOX conversion with this utility, use classic Outlook on Windows. Microsoft also documents bulk EML import in new Outlook, a separate option for messages already saved as EML; review its file-size and folder limits before choosing it. Opening an EML message is different from importing it into a mailbox.

Which route to choose

Choose direct conversion for a complete MBOX mailbox, IMAP to synchronize existing server mail or upload local messages through an authorized account, or EML for a small selection. The PST verification checklist covers folder counts, message content, and exceptions.

For a restoration test before migration, use backing up and restoring Thunderbird Local Folders. To inspect the archive without Outlook, see reading MBOX locally.

Primary sources

• Microsoft: CSV contact import and field mapping
• Mozilla: Thunderbird profiles
• Mozilla: Maildir in Thunderbird
• Google: Gmail in another email client

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