I don't know Outlook well enough, so I have to ask you what formats Outlook is able to accept. Then you can look at what tools are available to export and import messages in a suitable format. A quick bit of googling reveals several tools that allow eml messages to be imported into Outlook. This looks like an attractive route to me, as the .eml format preserves the message verbatim. But I think I'd look at setting up Outlook and Thunderbird alongside on the same computer, and see if you can drag-and-drop or copy-n-paste from one to the other. Or an IMAP-connected account, set up in both clients, could be used to transfer messages between the two. Copy or move into the IMAP account on Thunderbird, copy or move from the IMAP account in Outlook. It might not be terribly fast, but possibly the most direct and simple, since you do it all within the email environment, and so there is no issue with having to convert between formats and make decisions about attachments.