Import an MBOX archive into a Microsoft 365 mailbox

A PST that opens on your computer is not automatically part of your Microsoft 365 mailbox. To make that mail available on other devices, you need to copy or import it into the connected mailbox and let the upload finish. Follow these three steps with classic Outlook on Windows: convert, check, then import into your own mailbox.

Do you need a local archive or a server mailbox?

Microsoft 365 can mean an Office subscription or a hosted work mailbox. Having Word and Outlook installed does not identify the mailbox that should receive the archive. Write down the destination email address and confirm you can open that mailbox in a browser before starting.

Prepare a source copy and a destination

Keep the original MBOX files untouched. If the archive came from Google Takeout, extract every download part; if it came from Thunderbird, preserve the nested mailbox files. Use the Takeout guide or Thunderbird guide for those source-specific steps.

Check available destination storage and any restrictions on importing mail with the mailbox owner or administrator. A source archive's disk size is useful for planning but does not guarantee how much server storage the imported messages will use. Decide whether the destination needs all the old mail or only selected folders.

Start with a few messages you can recognize, including one with an attachment. Note their subjects and dates so you can find them again after the move. For a large or work archive, also use the full verification checklist.

Convert MBOX to PST and inspect it locally

Use the conversion instructions to create a new PST from the working source. Our MBOX to PST converter requires Windows and classic Outlook; it does not sign in to Microsoft 365 or upload mail itself.

Open the PST in classic Outlook with File → Open & Export → Open Outlook Data File. Inspect the test messages and folder tree before bringing them into the live mailbox. Resolve missing source folders or label duplicates at this stage. Uploading a flawed PST makes the same problem harder to untangle in the destination.

Import the checked PST into the right mailbox

Add the destination Microsoft 365 account to classic Outlook and confirm it is connected. Then use the Microsoft PST import procedure:

1. Choose File → Open & Export → Import/Export.
2. Select Import from another program or file, then Outlook Data File (.pst).
3. Browse to the checked PST and review the duplicate-handling option.
4. Select the source folder or archive root. Include subfolders when required.
5. Choose Import items into the same folder in and select the intended Microsoft 365 mailbox by its address.
6. Finish the import and leave Outlook connected while it synchronizes.

First use this procedure for the small test folder, or copy only the test messages. Verify them in webmail using the checks below before transferring the remaining folders. Record the test items so you can exclude them from a later bulk pass.

Read the destination selector carefully. Choosing an existing local data file will not put the messages on the server. For a few items, opening the PST and copying selected messages into a clearly named destination folder can be easier to check than importing the entire tree.

How do you know the upload really finished?

Open Outlook on the web in a separate browser session. Look inside the destination folders, find the recorded messages, and open their attachments. A message visible only in desktop Outlook has not yet proved that it reached the server.

Compare source and destination folder counts where they measure the same items. Allow for an intentional selection, pre-existing destination mail, and Gmail label copies. Record exceptions explicitly: for example, a source folder you chose not to upload. If a discrepancy remains, narrow it to a folder and date range rather than restarting the entire import.

What if the destination is incomplete?

Retain the original MBOX and checked PST until the server copy passes verification. Keep a record of the file imported, destination address, selected folders, and any unresolved messages. That record gives you a starting point if someone later reports a missing item.

When should an administrator handle the migration?

Use an administrator-led plan for many users, shared mailboxes, special retention requirements, or an archive that does not fit the intended mailbox. Microsoft provides a PST import service, but permissions, file preparation, and mailbox mapping are separate tasks. Do not treat a personal desktop import as a complete organization-wide migration plan.

Primary sources

• Microsoft: Import a PST into Outlook
• Microsoft: PST import service for Microsoft 365
• ReliefJet: MBOX to PST converter

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