Thunderbird Email Migration: Transfer MBOX & Local Folders to Microsoft 365 or Google Workspace

What Thunderbird Stores and Where

Everything is placed in a folder called a profile. The default location for the profile folder in Windows is C:\Users\<username>\AppData\Roaming\Thunderbird\Profiles\, and you can open the current one by going to Help > Troubleshooting Information and clicking the Open Folder button next to Profile Folder.

Two variants of data storage exist. Thunderbird stores information in the program profile, while Local Folders exist in the Mail\Local Folders directory within the profile and are often the only ones containing archived messages. By default, Thunderbird stores information in MBOX format, which is a single file containing many emails.

That’s why Local Folders deserve attention first. Re-adding a server account in the new environment brings back the server mail and leaves the archive behind.

Thunderbird Email Migration Checklist Before You Start

A few minutes of preparation prevents most failed uploads.

1. Close Thunderbird and copy the entire profile folder somewhere safe.
2. Create the destination mailboxes and determine which Thunderbird profile will be assigned to which user.
3. For Microsoft 365, make sure that the destination mailbox is available to receive transferred emails and that the required sign-in method is available.
4. Check mailbox sizes for individual users if you are migrating data manually with IMAP because Google imposes a limit on IMAP transfers per day.

Manual Method: Thunderbird Email Migration to Microsoft 365 or Google Workspace

The manual procedure makes use of the destination mailbox as an IMAP account in Thunderbird and copies the Local Folders needed there.

Thunderbird to Microsoft 365 Migration

1. In Thunderbird, add the Microsoft 365 mailbox as an IMAP account & sign in.
2. Once the server folders have appeared in the folder pane.
3. Expand Local Folders and select the desired folder.
4. Copy or move it to the newly created account and repeat the operation for all other folders.
5. Keep Thunderbird open until the upload finishes, then compare item counts in Outlook on the web.

For standalone MBOX files, an additional step may be needed to make the file accessible through Thunderbird’s Local Folders before copying the emails to the destination account.

Thunderbird to Google Workspace Migration

The process for Google Workspace is similar. Add the Gmail/Google Workspace mailbox as an IMAP account in Thunderbird and copy every necessary Local Folder into it. Gmail labels are represented as IMAP folders, so it is essential to test a deeply nested folder to see how the folder hierarchy is presented before migrating all emails.

Where the IMAP Method Slows Down

Google’s published bandwidth limits for IMAP uploads are 500 MB per day for a Google Workspace account. Their migration guidance discourages large copy/drag operations over IMAP and instead suggests using their supported migration option. The math on a 10 GB archive suggests roughly 20 days of uploads, if uninterrupted.

Other gaps show up in day-to-day work:

• Folder-level copying moves everything in a folder, with no date range or duplicate control.
• Standalone MBOX files need the workaround above before they can be copied using this Thunderbird-based method.
• Each profile needs its own IMAP setup, so effort grows with every user you add.

If your target is a desktop Outlook profile rather than a hosted mailbox, this walkthrough on how to convert Thunderbird emails to Outlook covers the PST route.

Migrate Thunderbird MBOX & Local Folder Data with DRS Softech

Once you need to transfer Thunderbird emails for multiple users or a large archive, the DRS Softech Thunderbird Backup Tool simplifies the process with features such as profile and MBOX selection, data preview, date filtering, custom folder names, duplicate detection, user mapping, and Skip Previously Converted Files. It supports migration to Office 365, Google Workspace, Gmail, or another IMAP account from a single wizard. 

Here’s the quick version, screen by screen.

Step 1: Add Source: Select the Thunderbird profile, MBOX files, or folder.

Step 2: Select Data: Preview the folders and apply filters or other required options.

Step 3: Choose Destination: Select Office 365, Google Workspace, Gmail, or IMAP and complete the sign-in and user mapping.

Step 4: Start Migration: Click Convert, monitor the status, and download the report.

Manual Method vs DRS Softech Tool

Task	Manual Method	DRS Softech Tool
Choosing what to move	Folder by folder	Profiles, MBOX files, or a whole folder in one screen
Standalone MBOX files	Placed in Local Folders first	Added directly
Date range	Not part of drag-and-copy	Date filter
Multiple users	One IMAP setup per profile	Fetched from Admin ID or CSV import
Progress tracking	Item counts compared by hand	Status, duration, and message count per user

Conclusion

A successful Thunderbird email migration means moving Local Folders and MBOX data completely, while preserving the folder structure and avoiding duplicates. The DRS Softech Thunderbird Backup Tool uses the interface to handle source selection, preview, filtering, user mapping, and delivery to Office 365 or Google Workspace from one wizard. Starting with a pilot run using the demo edition can help verify the migration setup before processing the full data set.

Frequently Asked Questions