Import emails PST from Windows 10 Outlook 2019 Office 365 to Mozilla Thunderbird 60

Scenario

User has emails stored in Windows 10 Outlook 2019 in Office 365 (called just "Outlook" in Windows 10) to Mozilla Thunderbird 60 also installed on Windows 10.

Pitfalls

Thunderbird has a convenient guide built in - hit F10, Tools->Import, select E-mails, then Outlook and then it runs. And then find out that it does not import all emails, in my case it only imported some empty folders and some Synchronization error mails. All the rest of the mails were not imported to Thunderbird.

Mozilla has written about the problems related to importing emails from Outlook to Thunderbird on their site and the problem is basically that Thunderbird cannot or is not good at talking to modern Outlook.

Therefore they suggest installing Outlook Express, export the mails from Outlook in a .PST file, import it into Outlook Express, set Outlook Express as the default e-mail client for the system and then run the import guide from Thunderbird. But this is not a solution for Windows 10 - because Outlook Express is deprecated and reliant on Internet Explorer and it refuses to install - if you manage to do it then Windows 10 tries to uninstall it automatically (if it is true is not verified).

Solution

Initially realize that Thunderbird can not import .PST files nor fetch emails from Outlook properly. These may both be hard to believe and they may of course change with newer versions.

What worked for me was to use MailStore Home which is some kind of free version of a mail archiving tool

Export the PST from Outlook, import it to MailStore Home and then export it from there to Thunderbird.

1. Find the import and export guide in Outlook and export each mail address as a .PST file.
2. Install MailStore Home - https://www.mailstore.com/en/products/mailstore-home/
3. Close Mozilla Thunderbird, Open MailStore Home.
4. Go to Archive E-mail, then under Mail Clients click on Outlook, then click Run.
5. Go to Export E-mail, then under Mail Clients click on Thunderbird, select the folders to import, use the default profile for Thunderbird, then click Run.