When the Import Goes Bad
Sometimes things can go when even with the most simplest of tasks. Perhaps you followed the above and yet, your older email files did not import as expected. There are several different ways that you can retrieve older emails from a previous client in the case that something has gone wrong.
Besides using the Import option in Thunderbird, you can also download different types of extensions. Extensions for Thunderbird are a bit like add-ons for Firefox - they make a change within the code of the email client so that you are able to extend usability for the program you are using. Thunderbird uses something call .EML or mbox files to save and store emails for the program; when you import your other emails into Thunderbird, it automatically changes the files (like the .pst used in Outlook) from the previous client into the mbox file.
If importing email into Thunderbird hasn't worked, you may want to try some of the extensions that will convert the emails. One popular extension is that of the ImportExportTools extension, which converts the emails in a friendly and easy way.
Depending on which email program you are using, even the operating system, you may need to follow a different way of importing email. Thunderbird has a listing of the different email programs an individual uses, as well as ways in which they can import the emails from these programs.
Importing email into Thunderbird is a fairly easy process which can allow for you to check all your email in one user friendly client program.