I found that moving from a Windows box using Outlook Express for email, to an Ubuntu box using Thunderbird was a little tricky. The export/import features took a bit of figuring out due to having multiple identities set up in Outlook Express.
Can’t Export from Outlook Express
The first problem that I came up against was no export facility in Outlook Express. The way I got around this was by installing Thunderbird for Windows on the Windows box alongside Outlook Express because Thunderbird has an import facility that would automatically pick up the Outlook data.
Thunderbird only Imports the Main Outlook Express Identity
That was the next problem. I had half a dozen email accounts set up as separate identities in Outlook Express, and Thunderbird only wanted to import the main identity.
I figured out that I could import the email from the main Outlook Express identity, find where Thunderbird stored it and save to USB. Then change the main Outlook Express identity and run the import again. Repeat as necessary until all of the email has been imported.
Move it to Linux!
On the Linux box run the Thunderbird profile manager and create multiple profiles as necessary. Or as I later decided, just have one identity and set up several accounts! Much simpler!
Then drop the email files into Thunderbird’s relevant directories.
Addresses Are Much Simpler
The address book was much easier to export/import. Just run the import in Thunderbird once and everything is copied over.