Export mail into Outlook (Express) or Apple Mail
From MozillaZine Knowledge Base
Apple Mail, Outlook Express, and Outlook cannot recognize Thunderbird's mailbox files in order to import them. If you try to import the mailbox files, you may get an error message such as "Import Error—The mail folder could not be opened. If another application is using this file, please close it and try again."
Below are some ways you can export your Thunderbird e-mail into Apple Mail, Outlook and Outlook Express based on actual users' experiences. Before trying any of these methods, it is recommended that you first try to get rid of all special characters in the names of mailbox folders (for example, "@", commas, and so forth). Otherwise, you might run into trouble later and might not get error messages even if something wrong does occur during the import process.
If and only if you have a small number of messages in your Inbox, you may also just select each message individually and click "File -> Save As -> File" so that the e-mail is now saved with an .eml extension, which you can then import into Outlook/Outlook Express or most other e-mail clients. If all else fails, you can still open the file with any text editor such as KWrite, gedit, or Notepad.
Using Netscape Communicator V4.7
You can use Netscape Communicator V4.7 to export email folders to Outlook(Express). Follow the following steps:
You can also use the MBOX autosave extension in Thunderbird to save your folders: Get MBOX Autosave
You can try importing your Thunderbird mail into Eudora, and then importing into your other mail client from there. However some users have reported that Eudora actually will not properly import Thunderbird messages with attachments or rich text formatting. (See this page for user discussion.)
NOTE: Because of differences between Eudora .mbox and Thunderbird's .mbox, all attachments will be lost by using this method
Using the ImportExportTools extension
The ImportExportTools extension supports exporting folders as either .eml or mbox files. While it might be laborious, it would be straight forward to export all of the messages in one folder, select all of the .eml files in Windows Explorer and drag and drop them to a POP accounts folder in a Outlook Express window, and then repeat as needed for the remaining folders. Outlook Express will import the .eml messages into the folder when you do that.
Outlook doesn't support dragging and dropping of .eml files but you could use Outlook Express as an intermediary, and then import them from Outlook Express into Outlook.
Older versions of Mail.App supported importing mbox files. See if the current version supports importing folders from a email client like Entourage that knows how to import mbox files as folders. If so, you could export the mbox files, import them in the intermediary email client, and then import them from that email client into Mail.App.
If using Windows Vista, where Outlook Express is not available, you would drag the .eml files to the Windows Mail preview pane. Outlook should then be able to import from Windows Mail.
You could also use third-party utilities such as mbox2eml, mailbag assistant, Aid4mail, IMAPSize, or mbx2eml to create .eml files and then drag and drop them into Outlook Express (plain Outlook won't work).
Eml To Pst Converter is a commercial application that will convert .eml files to a .pst file. You could export the messages as .eml files using the ImportExportTools add-on, connvert them using Eml to Pst Converter and then import the .pst file in Outlook. How to manage .pst files in Microsoft Outlook explains how to import the .pst file.
Using the Mailredirect extension
The Mailredirect extension supports forwarding one or more messages while making all of the important headers look like the headers in the original message. However you'd lose all status flags, you'd have to organize the messages into a folder hierarchy again, you'd probably have to do it in several steps to work around limits on your mailbox size, and have to deal with limits on how many messages your email provider lets you send within a hour. See this article for more information.
This works because IMAP lets you copy/move messages to/from the remote folders (on the mail server) as if they were local folders. It supports a client-server view of the world, not a download-centric one such as POP.
Depending upon your IMAP server you may lose some of the status flags such as 'replied to'.