Before importing into EagleFiler, make sure that Postbox has completely downloaded all the messages and use the Compact Folders command. The POP/local mailboxes are stored in the folder:
~/Library/Application Support/Postbox/Profiles/<profile>/Mail/Local Folders/
(To open the Library folder under Mac OS X 10.7 Lion, go to the Go menu in the Finder and hold down the Option key.) The IMAP mailboxes are stored in the folder:
~/Library/Application Support/Postbox/Profiles/<profile>/ImapMail/
The mailbox files have no extension and can be dragged from the Finder to EagleFiler. The .msf files are not mailboxes and can be ignored or deleted from EagleFiler’s library.
To import an individual message from Postbox, use the File ‣ Save As ‣ File command to save the message in Mail Files (.eml) format.