If you are switching to Thunderbird from another email application, chances are that you will be able to import emails from other emails clients. Thunderbird 1.5 lets you import emails from Netscape 4, Eudora, Microsoft Outlook, or Outlook Express.
In some cases, Thunderbird will not be able to import from another email client if it is running in the background. Make sure to first close the email program from which you are importing.
To import emails from another email client, go to Tools > Import, and the Import dialog will open. Choose the email program from which you want Thunderbird to import old emails, and click Next.
In our case, we chose to import emails from Qualcomm's Eudora email software. Thunderbird has automatically created a "Eudora Mail" as a sub-folder of our "Local Folders" folder.
Notice that Thunderbird maintained the hierarchy of Eudora's folders: the sub-folders of Eudora Mail are the folders we have setup in Eudora. Clicking the sub-folders reveals emails as if they had always been in Thunderbird. No glitch, or partial imports; all emails were transferred. That's it.
Transferring your emails from another email client to Thunderbird is usually a smooth process. However, keep in mind that the speed of the transfer depends on the amount of emails your are importing. Also, note that rich-content emails (that contain text formatting and graphics) are much larger than plain-text emails.