How to Migrate From Outlook to Thunderbird (2026)

Thunderbird shipped its 151.0.1 release on May 26, 2026, and the migration story has not changed in years: it still cannot read an Outlook PST directly, which trips up almost everyone who tries. I migrated a 12 GB Outlook profile across — IMAP mail, a 9,000-contact address book, and a stack of local-only archive folders — and the clean path is rarely the one Google’s first result suggests. Here is exactly what Thunderbird imports for free, what needs one add-on, and the gotchas that quietly drop mail if you skip them.

Why the PST Problem Exists

Outlook stores mail in a proprietary .pst file, and modern Thunderbird cannot read it. The old import wizard for Outlook was disabled in Thunderbird 38, so there is no one-click PST import. Migration instead routes around the PST using IMAP re-sync and standard formats like mbox.

This single fact is why so many migration attempts stall. People expect a “Import from Outlook” button, find none, and conclude the move is impossible. It is not — it just works differently. Per Mozilla’s own switching guide, the Outlook path in the import wizard was removed long ago, and Thunderbird now leans on open formats and live server sync instead.

The mental model that makes the rest easy: your mail lives in two places. Most of it sits on your provider’s server (Microsoft 365, Outlook.com, a company Exchange, or any IMAP host), and that copy re-downloads into Thunderbird automatically. Only the mail that exists solely inside the PST — old archives you dragged into local folders years ago — needs special handling. Sort your mailbox into those two buckets first and the migration plans itself.

Export Outlook to a PST

In classic Outlook choose File, then Open & Export, then Import/Export, then Export to a file, then Outlook Data File (.pst). Select the folders, include subfolders, and save. This PST is your safety net even when IMAP carries most of the mail across.

The exact path, straight from Microsoft’s export documentation:

In classic Outlook, click File.
Choose Open & Export, then Import/Export.
Pick Export to a file, then Next.
Select Outlook Data File (.pst), then Next.
Highlight your account or a specific folder, tick Include subfolders, and finish the wizard.

One trap to know up front: the new Outlook for Windows can only export mail, not contacts or calendar — Microsoft states that “calendar and contacts items saved in .pst files is not available in new Outlook at this time.” If you are on new Outlook, switch to the classic client for the export, or pull contacts and calendar out separately as CSV and ICS files. Make this PST before you touch anything else; even if you never import it, it is your rollback.

What Thunderbird Imports Natively

Thunderbird’s built-in importer, at Tools then Import, brings in address books (vCard, LDIF, CSV, TSV) and mail from clients like Apple Mail, Windows Mail, and Evolution. It does not import an Outlook PST or profile directly — that path was disabled in version 38.

Open the importer from the menu bar at Tools → Import. What it handles natively:

Address books — vCard (.vcf), LDIF, CSV, TSV, and tab-delimited text.
Mail from supported clients — Apple Mail, Windows Mail, Evolution, and SeaMonkey.
Settings and filters from those same clients — not from Outlook.

Outlook is conspicuously absent from that list, which is the whole reason this guide exists. So before you spend an evening fighting the importer, decide whether you even need it: if your Outlook account is IMAP-based, most of your mail will arrive through a simpler route. And if Thunderbird’s interface feels dated once you are in — many people find it does — a polished modern alternative like Mailbird imports the same accounts with a friendlier setup wizard, so it is worth a look before you commit. Thunderbird remains the free, open-source pick; Mailbird is the option if you will pay for a slicker UI. For a wider field, our roundup of the best email clients for developers compares both against the rest.

The IMAP Re-sync Shortcut

If your Outlook account uses IMAP, just add it to Thunderbird and every server-side folder re-downloads automatically — no PST, no import needed. This covers the bulk of most migrations and preserves read/unread state because that lives on the server.

This is the step that turns a dreaded migration into a 20-minute job. Add the account in Thunderbird and let it sync:

On first launch, enter your name, email address, and password.
Thunderbird auto-detects the server settings for most providers; choose IMAP (not POP) so mail stays mirrored on the server.
Let the initial sync run to completion — for a large mailbox this can take an hour, so leave it.

Because IMAP keeps the authoritative copy on the server, your Inbox, Sent, and every server folder reappear with read state and flags intact. POP would download and potentially remove mail from the server, so always pick IMAP for a migration. This is the same logic behind running Gmail and Outlook together in one client — the server is the single source of truth, and each client is just a view onto it. Only after this sync finishes should you worry about the PST-only leftovers.

Importing PST-only Mail With an Add-on

For folders that lived only inside the PST and never on a server, convert them to mbox or EML, then install the ImportExportTools NG add-on and use its Import function to load them into a Local Folder in Thunderbird.

These are the archives you dragged out of the server years ago to save quota. Thunderbird’s core importer will not touch them, but the ImportExportTools NG add-on does. The workflow:

In Outlook, the PST-only folders need to become standard files. Drag the messages back to an IMAP folder if quota allows (then they sync), or export the folder via a tool that writes mbox or individual EML files.
In Thunderbird, install ImportExportTools NG from the add-ons manager.
Right-click Local Folders, choose ImportExportTools NG → Import, and point it at your mbox or EML files.

The messages land in a Local Folder you control, fully searchable. This add-on is also the cleanest way to keep portable archives going forward — it exports any folder back out to mbox or EML in one click, which is far more future-proof than being locked inside a PST. If you are weighing Thunderbird against other free clients on exactly this kind of flexibility, the eM Client review covers how a commercial rival handles the same import job.

Moving Contacts and Calendar

Export Outlook contacts as CSV or vCard and import them through Tools then Import in Thunderbird. Calendar does not move through the mail importer — export each Outlook calendar as an .ics file and import it into the Thunderbird calendar, or connect by CalDAV.

Contacts and calendar travel on separate tracks from mail:

Contacts — in classic Outlook, File → Open & Export → Import/Export → Export to a file → Comma Separated Values, or export individual contacts as vCard. In Thunderbird, Tools → Import → Address Books and select the file.
Calendar — export each Outlook calendar to an .ics file, then in Thunderbird’s Calendar use Import and choose that file. If your account is on Exchange or a CalDAV host, skip the export and add the calendar as a network calendar so it stays live.

Verify a handful of contacts after import — field mapping between Outlook CSV columns and Thunderbird’s address book occasionally mislabels phone numbers or company fields, and it is easier to fix ten than nine thousand.

Common Gotchas

The migration drops mail or wastes time when you forget that filters, signatures, and Outlook categories do not transfer, when you choose POP instead of IMAP, or when you delete the PST before verifying counts. Check folder-by-folder before you trust the move.

The mistakes that bite, in order of how often I see them:

Choosing POP over IMAP. POP can pull mail off the server and break the re-sync safety net. Always IMAP for a migration.
Expecting filters and signatures to follow. They never transfer from Outlook. Budget an hour to rebuild rules and recreate signatures by hand.
Outlook categories vanish. Colored categories are an Outlook concept with no Thunderbird equivalent; tags are the closest substitute and must be reapplied.
Deleting the PST too soon. Compare message counts per folder in Thunderbird against Outlook before you delete anything. Keep the PST as a backup for a few weeks.
Forgetting the Sent and Drafts folders. People migrate the Inbox and overlook these — confirm they synced or were imported too.

Work through that list once and the move sticks. Skip it and you discover the gap weeks later when an archived thread is nowhere to be found. While you are tidying up, it is a good moment to stop the unsubscribe clutter from Outlook so you migrate a clean mailbox rather than years of newsletters.

Verdict

Migrating from Outlook to Thunderbird is straightforward once you accept there is no direct PST import: re-sync IMAP for the bulk, use ImportExportTools NG for local-only archives, and move contacts and calendar as CSV and ICS. Budget an hour, verify counts, and keep the PST as backup.

Best for: anyone leaving a paid Microsoft 365 plan they only used for mail, IMAP users who want a free open-source client, and people who value an importer that speaks open formats rather than locked PST files.

Skip if: you depend on Outlook-specific features Thunderbird does not replicate — deep Exchange calendar delegation, Outlook categories baked into your workflow, or tight Microsoft 365 group integration. In those cases a modern paid client may fit better than either.

Do the IMAP add first, confirm the bulk of your mail is in, then handle the local-only archives and contacts. Verify folder counts against Outlook before deleting a thing, and the 12 GB profile that looked daunting becomes an afternoon’s work.

Alexis Dollé

Founder & Editor

Alexis Dollé, email expert for 10+ years. Founder of Email Tools. I test every email client and utility myself, then write about them the way I’d explain them to a friend — no marketing fluff, no sponsored rankings, every claim sourced.

Sources & references

Microsoft Support, “Export or back up email, contacts, and calendar to an Outlook .pst file” — File > Open & Export > Import/Export > Export to a file > Outlook Data File (.pst); new Outlook exports mail only, not contacts or calendar. Accessed 2026-06-08. support.microsoft.com
Mozilla Support, “Switching to Thunderbird” — the import wizard is disabled for Outlook in Thunderbird 38 and newer; importer at Tools > Import; IMAP accounts re-sync server-side mail automatically. Accessed 2026-06-08. support.mozilla.org/en-US/kb/switching-thunderbird
Thunderbird 151.0.1 release notes — current monthly release dated May 26, 2026 (freshness reference). Accessed 2026-06-08. thunderbird.net
ImportExportTools NG, Thunderbird add-on — imports and exports mbox and EML message files into Local Folders. Accessed 2026-06-08. addons.thunderbird.net

Frequently Asked Questions

Can Thunderbird import an Outlook PST file directly?

No. Modern Thunderbird cannot read a .pst file natively — the old import wizard for Outlook was disabled back in Thunderbird 38. For mail that lives on an IMAP server, you do not need the PST at all because Thunderbird re-syncs it. For local-only PST folders, convert them to mbox or EML first, then load them with the ImportExportTools NG add-on.

Will I lose my emails when I switch to Thunderbird?

No, provided you migrate correctly. IMAP mail stays on the server and simply re-downloads into Thunderbird. The only data at risk is mail and contacts that existed only inside Outlook locally, which is why you export a PST and a contacts file first. Keep the originals until you have verified the counts match.

How do I import emails into Thunderbird from Outlook?

Three paths cover almost everything: re-sync IMAP folders automatically by just adding the account, import contacts via Tools then Import after exporting them from Outlook as CSV or vCard, and load any PST-only folders by converting them to mbox or EML and using the ImportExportTools NG add-on.

Does Thunderbird import my Outlook calendar and contacts?

Contacts import through the address book wizard once you export them from Outlook as CSV or vCard. Calendar does not move through the mail importer — export each Outlook calendar as an .ics file and import it into the Thunderbird calendar, or connect the account by CalDAV if your provider supports it.

Is Thunderbird really free?

Yes. Thunderbird is free and open source, funded by donations through MZLA Technologies, a Mozilla Foundation subsidiary. There is no paid tier and no per-seat licence, which is the main reason people switch from a paid Microsoft 365 plan when they only need mail.

What does not transfer automatically when I switch to Thunderbird?

Filters and rules, message signatures, custom views, Outlook categories, and the read/unread state of local-only mail do not carry over. IMAP read state is preserved because it lives on the server. Plan an hour to rebuild filters and signatures by hand after the mail itself is in place.

Related: Best email clients for developers — where Thunderbird and its rivals land. eM Client review 2026 — a commercial alternative with a smoother importer. Stop Outlook unsubscribe clutter — migrate a clean mailbox, not years of newsletters.