How to Migrate From Outlook to Thunderbird (Complete Guide)

Why People Switch From Outlook to Thunderbird

Microsoft Outlook has been the dominant desktop email client in corporate environments for over two decades. It integrates tightly with Exchange and Microsoft 365, offers robust calendar and contact management, and is a familiar part of the Office suite. However, a growing number of individuals and organizations are choosing to move away from Outlook in favor of Mozilla Thunderbird.

The reasons vary. Some users want to avoid recurring Microsoft 365 subscription costs. Thunderbird is entirely free and open source, with no licensing fees regardless of how many machines you install it on. Others prefer the privacy-oriented philosophy of Mozilla compared to Microsoft’s data practices. IT administrators at smaller organizations appreciate that Thunderbird works well with any IMAP or POP server, without requiring Exchange or Microsoft 365 infrastructure.

Thunderbird has also matured significantly in recent years. The Supernova redesign brought a modern interface, improved performance, and better extension support. For users who primarily need a reliable desktop email client without the overhead of the full Office ecosystem, Thunderbird is a compelling choice.

Whatever the motivation, the technical challenge remains the same: your emails are locked inside Outlook’s PST files, and Thunderbird cannot read them. Bridging that gap requires a format conversion.

The Format Challenge: PST to MBOX

Outlook stores its local email data in PST (Personal Storage Table) files. These are binary database files that contain emails, attachments, contacts, calendar items, and folder structures in a proprietary Microsoft format. The PST format is complex and undocumented outside of Microsoft’s own specification. No email client other than Outlook can open PST files natively.

Thunderbird, on the other hand, stores its email data in MBOX format. MBOX is a simple, text-based format where all messages in a folder are concatenated into a single file, separated by “From ” lines. It has been a standard in the Unix and open-source email world for decades.

To migrate from Outlook to Thunderbird, you need to convert your PST files to MBOX. This conversion must handle not only the email content itself but also folder hierarchies, attachments, message dates, read/unread status, and character encoding. A poor conversion tool can produce MBOX files that appear to import but contain corrupted messages, missing attachments, or broken folder structures.

Step 1: Export Your PST File

Before converting anything, you need to locate or export your PST file from Outlook.

If You Already Have a PST File

If you have previously exported your mailbox or if you are using Outlook with a local PST data file (common in Outlook 2013, 2016, and 2019), you may already have the file you need. PST files are typically located in:

C:\Users\<YourName>\Documents\Outlook Files\

You can also find the exact path by opening Outlook, going to File > Account Settings > Account Settings, then clicking the Data Files tab. The location of each PST file is listed there.

If You Need to Export From Outlook

If your email is stored on an Exchange server or in a Microsoft 365 cloud mailbox, you will need to export it to a PST file first. In Outlook, go to File > Open & Export > Import/Export, select Export to a file, then choose Outlook Data File (.pst). Select the mailbox or folders you want to migrate, choose a save location, and complete the export.

For large mailboxes, the export may take several minutes. Ensure you have enough disk space for both the PST file and the converted MBOX files that will be produced in the next step.

Step 2: Convert PST to MBOX With MailtoPst

With your PST file ready, the next step is converting it to MBOX format. MailtoPst handles this conversion entirely online, so you do not need to install any software or run any scripts.

Go to the PST to MBOX converter page on mailtopst.com. Upload your PST file by dragging it into the upload area or clicking to browse. MailtoPst accepts PST files created by any version of Outlook, including both ANSI (older, 2 GB limit) and Unicode (modern, up to 50 GB) PST formats.

Select MBOX as the output format. MailtoPst will parse the entire PST structure, extract every message with its headers, body, and attachments, and produce MBOX files that mirror your original folder organization. If your PST contains folders like Inbox, Sent Items, and custom subfolders, the converter will produce corresponding MBOX files for each.

Every new account starts with 100 MB free credit, which is sufficient for many personal mailboxes. Once the conversion completes, download the resulting MBOX files. If your PST contained multiple folders, you will receive a ZIP archive with one MBOX file per folder.

For users who need more background on PST files and their structure, see our guide on what a PST file is. If you want to handle the conversion without having Outlook installed at all, our article on converting PST to MBOX without Outlook provides additional context.

Step 3: Import MBOX Into Thunderbird

With your MBOX files downloaded, you are ready to import them into Thunderbird. Thunderbird does not include a built-in MBOX import feature in its default installation, but the widely used ImportExportTools NG add-on makes the process straightforward.

Install ImportExportTools NG

Open Thunderbird and go to Tools > Add-ons and Themes (or click the menu icon and select Add-ons and Themes). Search for “ImportExportTools NG” in the add-on search bar. Install the add-on and restart Thunderbird when prompted.

ImportExportTools NG is a well-maintained, open-source add-on with hundreds of thousands of users. It adds import and export capabilities for MBOX files, EML files, and other formats directly into Thunderbird’s menus.

Import Your MBOX Files

After the add-on is installed, right-click on Local Folders in Thunderbird’s folder pane (the left sidebar). Select ImportExportTools NG > Import mbox file. You will be given the option to import a single MBOX file or all MBOX files from a directory.

If you received a ZIP archive from MailtoPst, extract it first to a folder on your hard drive. Then use the “Import all mbox files from a directory” option and point it to that folder. Thunderbird will create a new folder for each MBOX file, preserving the folder names from your original Outlook mailbox.

Once the import is complete, your emails will appear under Local Folders in Thunderbird. You can then drag and drop folders to rearrange them, or move messages to your IMAP account if you want them synchronized with a mail server.

Verify the Import

After importing, take a few minutes to verify the results. Open several emails across different folders and check that:

• Message dates and sender information are correct
• Email body content displays properly, including HTML formatting
• Attachments are present and can be opened
• Folder structure matches your original Outlook organization

If everything looks correct, the migration is complete.

What Transfers and What Does Not

The PST-to-MBOX conversion preserves all standard email data: message headers, body content (plain text and HTML), file attachments, inline images, folder structure, and message flags (read/unread, flagged).

However, certain Outlook-specific features do not have equivalents in Thunderbird and therefore cannot be transferred:

• Calendar items and appointments: Outlook stores these in the PST file, but MBOX is an email-only format. You will need to export your calendar separately as an ICS file and import it into Thunderbird’s calendar or another calendar application.
• Contacts: Similarly, contacts stored in the PST file are not included in the MBOX conversion. Export your Outlook contacts as a CSV file and import them into Thunderbird’s address book.
• Tasks and notes: These Outlook-specific data types have no MBOX equivalent and must be migrated separately if needed.
• Outlook categories and color labels: Thunderbird uses its own tagging system, and Outlook category assignments do not carry over.
• Custom message flags: Outlook’s follow-up flags and reminders are proprietary and will not appear in Thunderbird.

For most users, the core email data is what matters most, and that transfers completely.

Tips for a Smooth Transition

A few practical suggestions can help make your Outlook-to-Thunderbird migration go smoothly.

Clean up before you convert. Delete spam, empty your trash, and remove any emails you no longer need. This reduces the size of the PST file and speeds up both the conversion and the import process.

Handle large mailboxes in batches. If your PST file is very large (over 5 GB), consider exporting specific folders or date ranges as separate PST files and converting them individually. This makes the process more manageable and makes it easier to verify results.

Set up your email accounts in Thunderbird first. Before importing your old messages, configure your IMAP or POP accounts in Thunderbird. This way, new emails will arrive in your Thunderbird inbox while you work on importing the historical archive.

Learn Thunderbird’s keyboard shortcuts. Many Outlook users have muscle memory for shortcuts like Ctrl+R (reply), Ctrl+Shift+M (new message), and Ctrl+E (search). Thunderbird uses similar shortcuts for most actions, but some differ. Spending a few minutes learning the differences will reduce friction in your first week.

Consider the Thunderbird-to-Outlook path. If there is any chance you may need to return to Outlook in the future, keep your original PST files as a backup. MailtoPst also supports the reverse conversion. For more on that workflow, see our page on Thunderbird to Outlook migration.

Start Your Migration

Moving from Outlook to Thunderbird does not require technical expertise or expensive tools. With MailtoPst, the format conversion that used to be the biggest obstacle is now a simple three-step process: export your PST, convert it to MBOX online, and import the result into Thunderbird.

Your emails, attachments, and folder structure will be waiting for you in Thunderbird, exactly as they were in Outlook.

Convert now — upload your PST file and get MBOX files ready for Thunderbird in minutes.