If you are looking to move all emails from one personal pc to a new one, from one e mail client to another one and perhaps from one operating-system to another you can actually get hold of a straight forward, realistic guidebook to copying your mail.
The transfer email manuals cost just $10 US. Guides are available from Thunderbird 3 to many other email clients, from Outlook express, from Windows Mail, all the main email clients. See if your wanted combination has been published.