No additional tools required.
This is a question I get from time to time from people who use Outlook (classic)1 (aka the big, powerful, Outlook email desktop client included with Microsoft Office/365/CoPilot) and are looking to switch to a different email program to run on their PCs. Typically, they’re moving to Thunderbird, but there are, of course, many options.
Getting your email from one to the other? Well, that takes a little effort.
Moving from Outlook's PST to Thunderbird
You can move email from Outlook to Thunderbird without buying a converter by using a free IMAP transfer account. Upload messages from Outlook to the transfer account and then download them in Thunderbird. Contacts and calendars require separate exports.
PST (and OST) formats
PST and OST are file formats used only by Outlook (classic) to store email. The formats are complex and proprietary (though they are documented), making moving your old email more difficult than it is between other email services.
There are third-party tools available to do this, both paid and free, of varying reputation and quality. For what is typically a one-time conversion, I’m reluctant to purchase anything. With just a little legwork and time, you can do the conversion yourself.
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The starting point: Outlook (classic)
The example above shows two email accounts I’ve configured in Outlook(classic), both connected via IMAP, and both whose email is stored in OST files.
Below them is “ExamplePST”, which is a PST file representing local storage on the PC only. It’s not associated with any email account.
In Account settings, you can see their respective locations.
It’s the contents of this PST file we want to move to Thunderbird.
Without using a third-party tool.
Step 1: Upload to a transfer account
There is no direct Outlook (classic) to Thunderbird copy. Instead, we need to use an intermediary email account to perform the conversion.
- Create an email account at any free provider that supports IMAP. I’ll use Gmail in my examples.
- Configure Outlook (classic) to connect to that email account using IMAP. Now your transfer account appears in the Outlook (classic) account list.
- Copy the email you want to move to that account. (I recommend copying rather than moving as a safety measure. If a copy fails, the original will not have been deleted.)
In the example above, I’ve selected all the messages in the folder “SavedNewsletters” in my ExamplePST and am about to drag them to the Inbox of my transfer account, askleotest2@gmail.com. NOTE the plus sign circled in red as part of the pointer icon: this indicates the operation will be a copy, as opposed to a move. Hold down the CTRL key and ensure that the plus sign appears before releasing the mouse pointer.
Because the transfer account is connected via IMAP, any email you copy into its folders in Outlook (classic) will be uploaded to the account online. You can confirm the upload by signing into the transfer account in your web browser.
The only “catches” here are:
- The upload time will vary depending on your internet speed and the amount of email you’re transferring.
- If you have more email to upload than the transfer account has room for — say the email provide limits you to a total of 5GB of email, but you have 20 GB to transfer — you may need to perform this operation in chunks.
Step 2: Download to Thunderbird
- Install Thunderbird if you haven’t already.
- Configure Thunderbird to access the transfer account using IMAP.2
- Let Thunderbird download all your email.
It’s that simple.
Now all the messages that were in your PST are available in Thunderbird.
Step 3: Organizing and cleaning up
Depending on your intent, there are several cleanup steps you might consider.
- Once you’ve copied your email out of Outlook (classic) to the transfer account, you might close and/or disconnect the PST file if you’re not going to use it anymore. Of course, I’ll suggest you back it up.
- Once you’ve uploaded all the email you plan to move to the transfer account, you can remove the transfer account from Outlook (classic).
- Once you’ve downloaded all the email you plan to move to Thunderbird, you may want to copy the email from the transfer account in Thunderbird to Local Folders (near the bottom of Thunderbird’s account and folder list on the left) to unlink them from the transfer account. This will prevent the email from being accidentally deleted locally if you delete it in the online account.
- Once you’ve got your email safely stored in folders unrelated to the transfer account, you can disconnect the account in Thunderbird.
- Once you’re done with everything, you can close or simply stop using the transfer account. You might delete any email still kept there that you’ve already downloaded to Thunderbird for safety/privacy’s sake.
Step 4: But what about…
There are two elephants in the room when it comes to moving things from Outlook to Thunderbird. (Actually, these are very common account management issues regardless of where or how you’re moving email data.)
Contacts. There’s no seamless way to move contacts other than to export them from one program and then import them into the other (Thunderbird, in our example). There will almost certainly be some form of data loss, as not all contact applications keep track of the same information in the same way.
Calendar. I have yet to encounter any reliable approach to transferring calendar information except for specific services (like Proton) being able to copy from specific other services (like Gmail). For calendar data stored in a PST, I’ve yet to find a viable option.
What about OSTs?
OST files represent an IMAP “window” on the master copy of email stored online. To move an OST file to Thunderbird, start at step 2 using whatever account you have in Thunderbird.
The difference is that when PST files are used, the email typically resides only in the PST, and we need a way to get it out. OST files are typically a copy of a master email repository that is stored and accessible elsewhere.
Alternatives to Thunderbird
You can move email from Outlook (classic) to any desktop email program you choose. I tend to prefer Thunderbird because:
- It’s free and open source.
- It’s quite powerful.
- It’s well supported by the Thunderbird community.
- It uses standard, open storage formats (unlike Outlook’s PST and OST files).
But any email client (ideally one supporting IMAP, as most do these days) will do. My only advice is to double-check that you’re not locking yourself into proprietary storage formats. When standard formats are used, you can avoid the hoops we just jumped through should you ever need to move to a different email program again in the future.
Do this
It’s understandable that people are migrating away from Outlook because of the changes that Microsoft has made to it. Getting your data out of proprietary storage formats like PST is a good first step.
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Footnotes & References
1: I’ll continually refer to this as “Outlook (classic)” so as to differentiate from the other programs also called Outlook which are nothing like the original Outlook email program that comes with (or came with) Microsoft Office 365. Only Outlook (classic), for example, deals with PSTs.
2: PST is a portable data file format. You can copy PSTs and open them in other instances of Outlook (classic) easily. They’re also the default format for POP3 connections. OST, while similar, is less portable in that it’s often encrypted and tied to a specific Outlook instance. OST is used for IMAP connections.
3: In theory, you could use POP3, but IMAP is safer in case anything goes wrong with a download.
