Import MBOX to Office 365 Without PST: Thunderbird to Office 365

Through a PST file, or straight into the mailbox

Besides the two ways compared below, an MBOX to Office 365 migration can be done by hand in Thunderbird itself. You connect it to the Microsoft mailbox over IMAP and drag the folders across. For that, Thunderbird has to sign in with OAuth2, because Microsoft has disabled Basic authentication for all organizations.

Two steps

MBOX to PST, then PST to the mailbox

A converter rewrites the Thunderbird archive as a PST. After that the PST still has to be imported.

Two programs and one more copy of the archive on disk
Microsoft's network upload for a PST asks for admin credentials
One vendor's guide warns that a repeated manual import can duplicate messages

One step

MBOX to the mailbox, with no PST made

It opens the MBOX and writes to the mailbox behind your sign-in.

No converter and no second copy of the mail
The sign-in is the mailbox user's
Writes into one mailbox per run

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Thunderbird to Office 365: your folders become mailbox folders

The tree of folders is carried over as it is. Point the program at your Thunderbird mail, and the folders in it are created in the mailbox under their old titles, nested the same way. The program creates real mailbox folders over IMAP, so IMAP access must be enabled for the mailbox. With IMAP disabled, the program stops with an error titled IMAP is turned off, and the fix is in the mailbox options or with your organization's administrator.

Nested folders keep their place in the tree
The program sends each message with its original date
Unread mail is passed on as unread
Stars and replied marks from Thunderbird are passed on too
Messages flagged as deleted in Thunderbird are not sent
Trash and Junk are unticked unless you tick them
Thunderbird tags are not transferred

Download for Free

How to import MBOX to Office 365 from Thunderbird in four steps

Add your Thunderbird mail folder

Download Email Migration Tool to a 64-bit Windows 10 or 11 PC and install it. If Thunderbird lived on another computer, copy its account folder over first. The screen has two panels, From on the left and To on the right. In From, click Add folder and choose the account folder. Do not be put off by the hint in the drop zone, which names EML folders only. Add folder takes a Thunderbird folder too. When the reading is done, you see how many messages were found. The files are only read.

Sign in to the Microsoft mailbox

In To, type the mailbox address. With an outlook.com, hotmail.com or live.com address the Email service field switches to Microsoft 365 / Outlook.com by itself. For an address on a company domain, pick that entry from the list. Then click the Sign in with Microsoft button and complete the steps in the browser window that opens. The consent screen there names the app Outlook Mail Merge, which is expected, and a work or school account may first need an administrator's approval. Once the To panel shows Connected, go on.

Tick the folders and choose where they go

Folders to migrate lists every folder with a checkbox. Select all and Clear all sit above the tree. The right side, Where messages will go, shows the result before anything is written. A folder the mailbox does not have yet is marked will be created. By default everything goes into Imported, inside a folder that carries the name of what you added, and you can remove that whole folder later if you change your mind. Switch on Merge into the mailbox folders to put Inbox into Inbox and Sent into the mailbox's sent folder instead. With Period set to Last 12 months, older mail is left for another time.

Start the migration and read the result

Click Start migration. While it runs, three counters move. Moved is what went up, Already in the mailbox, skipped is what was there before, Errors is what failed. A Stop button is there as well. At the end, Migration complete shows a table of what was in the files and what was moved. The rest is in Open migration log.

Four things it does for an MBOX archive

No conversion, the message goes up whole

Email Migration Tool does not convert anything when you import MBOX to Office 365. Inside an MBOX every message is a complete piece of mail already, headers and attachments included, and that is what the mailbox receives. Only the From-line escaping and the line endings are adjusted on the way.

One folder or the whole Thunderbird account

Add folder takes the account directory, or a single .sbd directory, and walks through everything inside. Thunderbird keeps each mail folder as a file with no extension and puts its subfolders into a directory with the same name plus .sbd. The .msf index files next to them are skipped. Need just one folder? Click Add files, switch the filter to All files (*.*) and pick that extension-less file. Its subfolders are found in the .sbd beside it. A Google Takeout file goes in through Add files as well. The download itself is a PST, MBOX and EML migration tool, and MBOX is the input this page is about.

Your password stays on Microsoft's page

What the program gets from the sign-in is a token for the mailbox, never the password. You type that on the Microsoft site, in your system browser. The token is held in memory and renewed quietly while a long run goes on. Close the program and it is gone. Next time you sign in again. If the sign-in fails, the Test button shows which stage broke.

A dropped link does not double your mail

If the link breaks, the program tries to reconnect up to 5 times, waiting from 2 to 30 seconds between tries. When the mail service tells it to slow down, it waits 30 to 120 seconds and tries again. There is no daily quota counter for Microsoft in the program, and it does not know the limits of Exchange Online. A longer break is covered too. After each message the program notes its progress in a journal on disk, and if you close it mid-run, the next launch offers Resume. Before sending, it looks at the destination for the same Message-ID. When there is no ID, it matches on sender and subject together with size and sending time. It is designed to pass over what is already there. Duplicates that were in Thunderbird to begin with are not removed.

Who uses it to import MBOX to Office 365

Teams moving off Thunderbird

Five people, five Thunderbird profiles, and Microsoft 365 starting next month. Nobody has to collect the archives in one place. Each person runs the program on the PC where the profile lives and signs in to their mailbox.

New hires with mail from a previous address

You joined a company that runs Microsoft 365 and brought years of correspondence with you, exported as MBOX. Select only what belongs at work. Private mail from that file may fit a personal mailbox better. If that one is at Google, see how to import an MBOX file into Gmail with the same program.

Whoever moves 2 or 3 mailboxes by hand

Not a migration project. An afternoon and two or three colleagues. Do the mailboxes in turn, with a new migration for each. There is no bulk mode that takes a list of mailboxes.

When an MBOX archive needs a Microsoft mailbox

A small post office with its shutter half down and sacks of letters waiting outside.

The mail hosting plan is ending

The provider that hosted your domain's mailboxes named a closing date, or you canceled the plan yourself. After the closing day the server copy is gone. What Thunderbird downloaded over the years is all there is. Put it into the new mailbox while the old one still answers and you can compare the two.

Two desk mail trays, an old wooden one full of letters and a new empty one beside it.

Outlook replaces Thunderbird

Maybe it is a new policy, maybe just the calendar the others use. The mail client changes next week. Thunderbird 145 and later can talk to Microsoft 365 natively, but that only sets up the connection. Mail kept locally remains on the PC until something puts it into the mailbox.

A silver laptop and a desktop tower with a USB stick lying between them.

The old mail is on a Mac

Someone on the team used Apple Mail. Exporting a mailbox there gives an .mbox package. Copy it to the Windows PC and hand it over with Add folder. One package becomes one folder. There is no Mac version of the program.

A wall of empty pigeonholes in an office and a cart of bundled old letters in front of it.

The new mailboxes are ready and empty

The licenses are paid and new mail arrives at the new addresses. All mail from before this month is still in Thunderbird, on a PC that someone leaves switched on just for that.

Upload MBOX to Office 365 on trial first

The trial is the full program with one cap.

For each MBOX folder	Trial	Licensed
Messages sent to the mailbox	The latest 25	Every one
Thunderbird folders, Apple Mail packages, Takeout files	✓	✓
Expiry	Never	Never
Messages sent on trial are real ones, filed under Imported. Before the start, a window titled Trial version tells you how many messages of a folder will be moved and how many wait for a license. Later, with a license key entered, Move the rest sends what the trial left behind. The key is activated on one computer and works there.	Download	Buy now From $ 24.99

Buy now

From $ 24.99

Download

Messages sent on trial are real ones, filed under Imported. Before the start, a window titled Trial version tells you how many messages of a folder will be moved and how many wait for a license. Later, with a license key entered, Move the rest sends what the trial left behind. The key is activated on one computer and works there.

Email Migration Tool for Gmail and Office 365 - Import PST, MBOX and EML

Price

from $ 24.99 / month

Version

3.3

File Size

67.7 Mb

Last updated on

14/08/26

Trial: 25 messages from every MBOX folder, newest first, with no end date

System Requirements

Windows 11/10/8.1/8/7 (32/64 bit)
Intel i3, AMD Ryzen 5 or above
4 GB of RAM or above
NVIDIA® GeForce® series 8 and 8M, Intel® HD Graphics 2000, Quadro FX 4800, Quadro FX 5600, AMD Radeon™ R600, Mobility Radeon™ HD 4330, Mobility FirePro™ series, Radeon™ R5 M230 or higher graphics card with up-to-date drivers
1280 × 768 screen resolution, 32-bit color
1 GB of free hard disk space or above

Frequently Asked Questions

How can I migrate my MBOX to Office 365?

Load it with a desktop program that logs in as the mailbox user and puts the mail there itself. With Email Migration Tool that is four steps on one screen, shown above. The longer ways are a converter that produces a PST for a later import, or dragging folders inside Thunderbird after the Microsoft account is added to it. Pick the direct way when the mail is on your PC and the mailbox is yours to sign in to.

How do I import an MBOX file?

Add it to a program that reads MBOX and tell that program where the mail should go. With Email Migration Tool a single .mbox or .mbx file is added through Add files and becomes one folder named after the file. The format is detected from the content, not from the extension, which helps when the file has an odd name.

Will Outlook open MBOX files?

Not directly, going by Microsoft's own import steps. They name the Outlook Data File (.pst) and no MBOX option, which is why the usual advice starts with a converter. If your old mail is already in a PST, the PST to Office 365 migration tool page covers that file. With an MBOX you can leave Outlook out of the job. Once the messages are in the mailbox, you read them in Outlook or on the web like other mail.

Can I import MBOX to Outlook?

Yes, if by Outlook you mean an Outlook.com mailbox and not the desktop app. An address at outlook.com, hotmail.com or live.com is a Microsoft mailbox, and Email Migration Tool is built to sign in to it with the same Sign in with Microsoft button as for Microsoft 365. Type the address in To and the service is picked for you.

How to import MBOX to Microsoft 365?

To import MBOX to Microsoft 365 you do not need the admin center, only the mailbox user's sign-in. The target has to be a regular mailbox. Shared mailboxes and archive mailboxes are not supported. When the server refuses a message as too big, the run goes on without it and the report says which one.

Does it work with my work account?

Email Migration Tool is built to connect a work account, as long as your organization lets you approve the sign-in. The app on the consent screen is called Outlook Mail Merge, because the program uses a sign-in registration shared with another product of the same publisher. Your organization may also want an administrator to agree first. The program then shows the error Administrator permission is required, and the same window offers a ready note for your administrator. Whether your organization asks for that is its policy, and the trial is the quick way to find out.

Does it move my calendar and contacts?

No, it moves mail only. An MBOX file holds messages and nothing else, so there is no address book or calendar in it to move. Mail rules are not transferred either.

Can Thunderbird be used with Microsoft 365?

Yes. Thunderbird's own blog announced native access to Microsoft 365 mail, and you can keep the client you know after the move. Adding the account is one job and moving the archive is another. The account shows what is on the server. Folders you kept under Local Folders are files on this PC, and connecting the account does not send them anywhere. Email Migration Tool does that part. It reads those files and adds the messages to the mailbox.

Import MBOX to Office 365 in One Step, No PST Conversion