Like me, if you are dying to try out an alternative to Entourage, but are stuck because all your previous mail is in Entourage, here’s what to do. Click on the Folder list in the left-hand pane and drop it onto your desktop; this will export the folder in the popular mbox format.
Next, use your email program to import the mbox format. In the case of Thunderbird, this has to be done manually at the moment. To accomplish this, open the ~/Library -> Thunderbird -> Profiles -> default -> random_string.slt -> Mail -> Local Folders folder. Drop the mbox files in there and you’re set.
The same works for Mozilla, where the mail folder resides in the ~/Library -> Mozilla -> Profiles -> default -> random_string.slt -> Mail -> Local Folders folder.
The same technique should exist for apple mail.
.mbox file also work at Firefox for windows, juts put you .mbox file to Thunderbird profile mail “C:\Documents and Settings\user\Application Data\Thunderbird\Profiles\7lhh4epl.default\Mail\Local Folders”
Good luck trying!