Why does Microsoft 365 Reject MBOX Files?
Microsoft 365 mailboxes accept PST data, not MBOX files. You either convert MBOX to PST first or use a tool that saves data into the mailbox directly.
| Method | Steps | Suits |
|---|---|---|
| Convert to PST, then Outlook import | Convert, open Outlook, import | One or a few mailboxes |
| Convert to PST, then Purview Import | Convert, upload, map | Admins with many mailboxes |
| Direct migrator tool | Select files, map users | Direct import with no PST step |
How do You Import MBOX With Outlook?
Method #1: Convert to PST, Then Import in Outlook (Paid After Demo)
Who it suits. Users with a few MBOX files. Extra application. MBOX Converter and Outlook.
- Install the MBOX Converter and add the MBOX files or the folder.
- Select PST as the export format, set a date filter if needed and save the file.
- Open Outlook with your Microsoft 365 account.
- Select File, Open & Export and Import/Export.
- Select Import from another program or file, then Outlook Data File (.pst).
- Choose the PST file and the mailbox folder, then select Finish.
The converter previews messages, takes many MBOX files at once and can split output by size. The trial exports 25 items per folder.
How do Admins Import PST Files in Bulk?
Method #2: Purview Import Service (Free)
Who it suits. Admins who move many mailboxes. Extra application. AzCopy.
Microsoft documents network upload as the free option. Upload the PST files to a temporary Azure location with AzCopy, then create an import job in the Microsoft Purview portal and submit a CSV mapping file.
Plan around these limits. Microsoft recommends 20 GB or less per PST. Items over 150 MB are skipped.
The import rate is about 24 GB per day per mailbox. The admin needs the Mailbox Import Export role or Global Administrator rights.
Can You Avoid the PST Step?
Method #3: MBOX to Office 365 Migrator (Paid After Demo)
Who it suits. Admins who want MBOX data in the mailbox with no PST file. Extra application. MBOX to Office 365 Migrator.
The MBOX to Office 365 Migrator imports MBOX, MBS and MBX files into Office 365 user or admin accounts. It moves many files at once, imports deleted emails from MBOX files and saves a final report as CSV. The setup needs an Office 365 console project, as the vendor guide explains.
What should You Check Before the Import?
- Copy the MBOX files to a safe folder.
- Check the mailbox size against the Exchange mailbox size limits.
- Test with one file and compare folder counts.
- Remove duplicates before you import.
Source files from Thunderbird or Apple Mail work too. Our MBOX file guide explains the format, and the guide to open MBOX files in Outlook explains the desktop route.
Frequently Asked Questions
Can Microsoft 365 Import MBOX Files Directly?
No. Convert MBOX to PST first, or use a migrator that saves data into the mailbox.
How do You Import the PST After Conversion?
Open Outlook, select Import/Export, choose Import from another program or file and select the PST.
What PST Size Suits Purview Import?
Microsoft recommends 20 GB or less per PST. Larger files can slow the import.
What Happens to Items Over 150 MB?
Microsoft states that the Import service skips items larger than 150 MB.
Which Role does the Admin Need?
The Mailbox Import Export role in Exchange Online, or Global Administrator rights.
Does the Migrator Import Deleted Emails?
The vendor page states that it imports deleted emails from MBOX files.
Can I Import MBOX From Thunderbird or Apple Mail?
Yes. The converter and the migrator accept MBOX files from those apps and from Google Takeout.
Author's Verdict
Convert to PST and import with Outlook for a few files.
Use Purview Import for bulk PST moves and a migrator when you want no PST step.
Key Takeaways
- Microsoft 365 needs PST data.
- Limit every PST to about 20 GB.
- A migrator skips the PST step.
Related Guides
Opening the MBOX file in desktop Outlook? Read how to import an MBOX file into Outlook.