Converting messages from Thunderbird to Outlook
Instructions taken from: http://www.broobles.com/imapsize/th2outlook.php
1. Compact your Thunderbird folders (optional)
Thunderbird keeps messages in mbox files even after they have been deleted. If you don’t want to include deleted messages in the conversion, you have to compact your folders in Thunderbird. Right click a folder and select “Compact This Folder”. This will completely remove deleted messages from the folders mbox file.
2. Locate mbox files in Thunderbird
Thunderbird stores messages in the Profile Folder, in the Mail and (if you use IMAP) ImapMail folders. Each mail folder (Inbox, Sent, etc.) is stored as two files — one with no extension (e.g. INBOX), which is the mail file itself (in “mbox” format), and one with an .msf extension (e.g. INBOX.msf), which is the index (Mail Summary File) to the mail file.
3. Convert mbox files to eml files
IMAPSize has a built-in mbox2eml tool that can seamlessly convert thousands and thousands of messages. Download, install and run IMAPSize (it’s free: http://www.broobles.com/imapsize/download.php) and invoke the mbox2eml option from the Tools menu. Select the mbox file you wish to convert (you might need to set the “Files of type” option to “All Files” in the file selection dialog, since Thunderbird mbox files don’t have an extension) and select a location where generated eml files should be stored. Click “Convert” and IMAPSize will start converting the files.
All the attachments will also be converted and your original Thunderbird files will not be touched. Starting from version 0.2.0. you can convert multiple mbox files at once.
4. Import generated eml files into Outlook Express
Launch Outlook Express and select the local folder (or create a new one) into which you want to import your messages. Open Windows Explorer or your favorite file manager, locate the generated eml files, select them and drag and drop them into the OE application window. If Outlook Express is your email client of choice you don’t have to perform the next step.
5. Export files from OE to Outlook
Assuming you have Outlook installed on your system, open Outlook Express and from the File menu select Export/Messages. Once exported, the messages will appear in Outlook – if they were in your inbox in OE, they’ll be added to your Outlook inbox; if they were in a local subfolder in OE, a folder of the same name will be created under your Outlook inbox.
I was using Outlook 2007 and Thunderbird v126.96.36.199 on a Windows XP Pro SP3 machine.