How to Convert Thunderbird to Microsoft 365

  

Nick Rogers
Published: May 13, 2026 • Convert • 6 Min Read

Why migrate from Thunderbird to Microsoft 365?

Thunderbird is a capable free desktop client. However, it stores mail locally and misses the cloud features that growing teams need. As a result, you lose cloud calendars, Teams integration, OneDrive storage, and central admin controls.

Microsoft 365 fixes those gaps. Every mailbox gets a 50 GB to 100 GB cloud quota. In addition, you get Exchange Online server-side rules, shared calendars, and access from Outlook on the web, mobile, and desktop without manual sync.

The migration itself is not hard. Thunderbird stores each folder as a standard MBOX file. Microsoft 365, however, does not accept MBOX natively. Therefore, you need an intermediate step.

Before you start: prerequisites

• An active Microsoft 365 mailbox (Business Basic, Business Standard, or any Exchange Online plan).
• Thunderbird installed with your existing local folders intact.
• A recent backup of your Thunderbird profile folder. On Windows: %APPDATA%\Thunderbird\Profiles\. On macOS: ~/Library/Thunderbird/Profiles/.
• Outlook desktop (2016 or newer) if you plan to use Method A or Method C.
• Admin permission to add the Microsoft 365 account to Outlook.

Step 1: Export Thunderbird folders to MBOX

Thunderbird stores mail in MBOX format internally. However, the easiest way to extract clean, named files is the free ImportExportTools NG add-on.

1. Open Thunderbird, then go to Tools > Add-ons and Themes.
2. Search for ImportExportTools NG, then click Add to Thunderbird.
3. Restart Thunderbird when prompted.
4. Right-click the folder you want to migrate (for example, Inbox) and choose ImportExportTools NG > Export folder.
5. Pick a destination on your desktop. The folder is saved as an .mbox file.
6. Repeat for every folder you need to move, including Sent, Drafts, Archives, and any custom labels.

For large mailboxes over 5 GB, export folders one at a time rather than the whole account. As a result, you avoid memory errors and the next step runs faster.

Step 2: Choose your import method

There are three reliable ways to move Thunderbird mail into Microsoft 365. Pick the one that matches your mailbox size and technical comfort.

Method A: Drag-and-drop through Outlook desktop (best for under 10 GB)

1. Add your Microsoft 365 account to Outlook through File > Add Account, then let it finish syncing.
2. In Thunderbird, keep the source folders visible.
3. Arrange Thunderbird and Outlook side by side.
4. Select messages in Thunderbird (Ctrl+A for the whole folder), then drag them into the matching Microsoft 365 folder in Outlook.
5. Outlook uploads the messages to Exchange Online in the background. Watch the send/receive status bar.

This method preserves dates, attachments, and sender details. However, it does not migrate calendar items or contacts. Therefore, those need a separate export from Thunderbird’s address book.

Method B: IMAP sync between Thunderbird and Microsoft 365

1. In Microsoft 365 admin or your Outlook on the web settings, confirm that IMAP is enabled for your mailbox.
2. In Thunderbird, go to File > New > Existing Mail Account, then add your Microsoft 365 address. Thunderbird auto-detects the Microsoft 365 IMAP server (outlook.office365.com, port 993, SSL).
3. Once the account appears in the left pane, drag folders or individual messages from your Local Folders tree into the new IMAP account.
4. Thunderbird uploads each message over IMAP, which can take several hours for large mailboxes.

This is the simplest method for small migrations. However, it can be slow for mailboxes above 10 GB. In addition, Microsoft 365 occasionally throttles uploads when it detects a high append rate.

Method C: Convert MBOX to PST and import (best for 10 GB and above)

For bulk migrations, the most predictable route is to convert the exported MBOX files into a single PST. Then use Outlook’s Import/Export wizard.

1. Use a trusted MBOX to PST converter to turn each .mbox file into a PST. Verify the tool preserves folder hierarchy and attachments.
2. Open Outlook desktop signed in to your Microsoft 365 account.
3. Go to File > Open & Export > Import/Export > Import from another program or file > Outlook Data File (.pst).
4. Browse to the PST, choose Do not import duplicates, and select the Microsoft 365 mailbox as the destination.
5. Outlook copies the messages into Exchange Online. The first sync may take a few hours for large PSTs.

If you need to import a PST file to Microsoft 365 in bulk for multiple users, the Microsoft 365 Network Upload service is a free alternative for admins.

Step 3: Verify the migration

After the import finishes, spot-check the results before you decommission Thunderbird.

• Compare folder counts in Outlook on the web against your Thunderbird source.
• Open three or four older messages and confirm attachments open correctly.
• Check that the sent date matches the original, not the import date.
• Search for a known keyword to confirm Microsoft 365 has indexed the new content.

Common errors and fixes

“Cannot create new file” in Outlook

The PST is over 50 GB. Therefore, split it into smaller PSTs before importing.

Attachments missing after IMAP transfer

Microsoft 365 rejects attachments above 150 MB. As a result, you need to save oversized files to OneDrive and link them from the message.

Sent dates show today’s date

This happens when you used Outlook’s Move instead of Import. Therefore, re-import from the original MBOX with the Import wizard to preserve timestamps.

Throttled uploads

Microsoft 365 limits the IMAP append rate. If Thunderbird shows repeated “server busy” errors, pause for 15 minutes, then resume.

What about contacts and calendars?

Thunderbird stores contacts in an address book and calendar items in the Lightning add-on. Neither moves with the MBOX export. However, both are easy to migrate separately.

• Export contacts to LDIF or CSV from Address Book > Tools > Export, then import the CSV into Outlook.
• Export calendars to ICS from the Lightning calendar pane. Then import the ICS into Outlook on the web under Calendar > Add calendar > Upload from file.

Related guides

• Import PST file to Microsoft 365
• Backup Google Workspace emails
• Extract email addresses from Thunderbird
• Transfer emails from PC to Mac

Final thoughts

Moving from Thunderbird to Microsoft 365 is a one-time effort that pays back every day. You gain cloud access, mobile sync, and team collaboration. First, pick Method A for small mailboxes. Next, choose Method B if you are comfortable waiting on IMAP. Finally, use Method C when you need to migrate years of mail in a single pass. Whichever route you choose, keep a copy of your original Thunderbird profile until you have verified every folder on the Microsoft 365 side.