How to Transfer Emails from Outlook to Thunderbird (2026)

To transfer emails from Outlook to Thunderbird, choose the route that matches where your mail is stored. If it lives in a Microsoft 365, Outlook.com or IMAP mailbox, add that account to Thunderbird and let it sync. If it sits in local Outlook data, use Thunderbird's Import tool (≡ > Tools > Import). A bare .pst file needs a detour through Outlook or a converter, as the sections below explain.

Key takeaways

• Work out where the mail is stored first. A mailbox on a server needs no file transfer, only a new account in Thunderbird.
• Local Outlook data goes through the Import tool; a bare PST needs Outlook or a converter.
• For Microsoft 365, the protocol choice (Graph, EWS or IMAP) depends on your Thunderbird version and your tenant's settings.
• Contacts move as a CSV file and calendars as an ICS link or file.

Which method fits your situation?

Pick the row that matches where Outlook keeps your mail.

How do I transfer emails with Thunderbird's Import tool?

Use this route when the mail is in local Outlook data. Mozilla's import article describes the tool as a way to bring accounts, messages, address books, calendars and settings into Thunderbird from other Thunderbird profiles, other applications or certain files. Which of those you are offered depends on the source you pick.

1. Confirm Outlook is installed. Mozilla states that Outlook must be installed on your device; a .pst file alone does not work.
2. In Thunderbird, open the menu and choose ≡ > Tools > Import.
3. Select Outlook in the list of import sources and click Continue.
4. Mozilla describes the rest as three steps: specify the source, choose which parts of the data to import, then confirm with Start Import. Select the mail, and the address book if it is offered.
5. When it finishes, compare folder names, message counts and a few attachments with Outlook before you remove anything.

Treat the import as a copy job. Keep the Outlook data until you have checked the result, and do not import the same mailbox again later through account sync as well, or you may end up with duplicates (this is our advice, not a Mozilla statement).

How do I move a Microsoft 365 or IMAP mailbox into Thunderbird?

Mail that already sits on a server does not need exporting. Add the account and Thunderbird downloads it. Mozilla's Thunderbird 153 release post says the new Account Hub offers improved autodiscover, automatic protocol detection and automatic setup of connected calendars, and scans for address books you have not yet configured.

For Microsoft 365 there are three protocol choices, and Mozilla's September 2026 post on Microsoft Graph explains the timing:

• Microsoft Graph. First enabled in the Thunderbird 154 Release channel, for Microsoft 365 accounts only. Mozilla says it supports email only so far, not calendar or address book. The 153 ESR does not support Graph.
• Exchange Web Services (EWS). Native Exchange support in 153, no add-on needed. Mozilla says Microsoft will begin a phased shutdown of EWS on Microsoft 365 and Exchange Online starting in October 2026, with the goal of a complete shutdown in 2027. EWS stays supported for on-premises Exchange. If you set up an EWS account, Mozilla's advice is to delete it and add it again through Account Hub, choosing the Graph option.
• IMAP. Mozilla says that if IMAP currently works for an on-premises Exchange or Microsoft 365 account, you need to do nothing.

If you choose IMAP, use the server details Microsoft publishes:

Sources: Microsoft Learn on POP3 and IMAP4 in Exchange Online and Microsoft Support on Outlook.com server settings, which says both IMAP and SMTP require OAuth2 modern authentication.

Two Microsoft notes matter for work accounts. IMAP4 is enabled by default for Exchange Online users, but security defaults switch it off. Disabling basic authentication blocks legacy protocols such as POP and IMAP. If the sign-in fails, ask your administrator before you change anything on your side.

Microsoft also says IMAP clients can create and access multiple folders on the server, so your folder structure arrives with the sync. We have not found a vendor statement that read and unread status always carries over, so check a sample folder before you rely on it.

What if I only have a PST file?

A PST is Outlook's own data file, and Thunderbird will not open it directly. You have two practical options.

1. Go through Outlook. Add the file to Outlook, then move the folders into a mailbox that Thunderbird can sync. In classic Outlook, Microsoft's PST import page gives the path as File > Open & Export > Import/Export, then Import from another program or file and Outlook Data File (.pst).
2. Convert it. Some third-party converters say they turn a PST into mbox or EML files. We have not tested any, so try one on a small copy first. Thunderbird's own ImportExportTools NG add-on, per its project page, imports and exports mbox and EML files and lists support for Thunderbird 140 to 157. It makes no mention of PST.

New Outlook. Microsoft's pages disagree here. Its PST import page says new Outlook for Windows can import a .pst into a mailbox (Settings > Data files > Import) or open it without importing (Settings > Data files > Outlook Data Files > Add file). Opening needs classic Outlook installed on the same PC, the same 32-bit or 64-bit build of both, and a Microsoft 365 subscription on the account, and Outlook Data Files are not supported on the ARM version of new Outlook. Its PST export page says importing a mailbox or calendar from a .pst is currently not supported in new Outlook. Check which applies to your build, or use classic Outlook.

If you need to create a PST from Outlook first, the same export page gives these paths:

• Classic Outlook: File > Open & Export > Import/Export > Export to a file > Outlook Data File (.pst), then pick the account and keep Include subfolders ticked.
• New Outlook: Settings > Files > Export (the import page calls this area Data files). Microsoft adds that contacts and calendar items saved in .pst files are not available in new Outlook at this time.

Microsoft says an export includes attachments but not folder properties, message rules or blocked sender lists. It also warns that in Cached Exchange Mode, Outlook exports only the items in the cache, which by default covers the past 12 months. If older mail seems to be missing, that is the first thing to check.

How do I move contacts and calendars?

Contacts. If they live in a Microsoft 365 account, Mozilla says Account Hub scans for available address books you have not yet set up. The Graph support in the 154 Release channel does not cover address books yet, so a file is the portable fallback. Microsoft's contact export page gives these CSV paths:

• Classic Outlook: File > Open & Export > Import/Export > Export to a file.
• New Outlook: People > Export contacts.
• Outlook on the web: People > Manage contacts > Export contacts.

Thunderbird's address book can usually take a CSV file. The menu differs between versions and we have not verified it, so check Thunderbird's own help.

Calendars. From Outlook on the web, open Settings > Shared calendars, choose a calendar under Publish a calendar and select an ICS link, as described on Microsoft's calendar sharing page. Microsoft notes that ICS calendars are read-only and that how often they sync depends on the recipient's email provider. Thunderbird can subscribe to an ICS link or import the downloaded file; we have not verified the menu path, and a subscribed copy updates only as often as the provider refreshes it. For a Microsoft 365 account, the 153 release post says Account Hub can set up connected calendars automatically.

Can I go from Thunderbird back to Outlook, or run both?

The lowest-risk trial is to point both programs at the same mailbox, so nothing is moved. For files, classic Outlook's Import/Export wizard accepts .pst files (Microsoft's PST import page), and Microsoft's calendar import page shows an option for iCalendar (.ics) and vCalendar (.vcs) files. We found no mbox option in the Microsoft pages we checked, so mbox exported from Thunderbird would need conversion first.

Why does the transfer fail?

FAQ

Can Thunderbird open a PST file?

Not directly. Go through Outlook, or use a converter you have tested to produce mbox or EML files, then import them with an add-on such as ImportExportTools NG.

Will folders and read status transfer?

Folder structure should, because it lives on the server. Read status is not documented by the vendors we checked, so spot-check one folder.

Does this work with the new Outlook?

Mail already on a server does, because Thunderbird syncs the account rather than reading Outlook. For PST files, Microsoft's pages disagree on whether new Outlook can import one, so check Microsoft's current guidance for your build.

Will IMAP keep working with Microsoft 365?

Only while your tenant allows it, since Microsoft says disabling basic authentication blocks legacy protocols such as IMAP. Mozilla's September 2026 post says an IMAP account that works today needs no change.

Moving a whole team, or a mailbox with years of local archives, is easier with a plan. Our helpdesk team handles email management and Microsoft 365 administration, and can run a migration through remote IT support. If you are still deciding whether to switch, read why people use Thunderbird as their email client.