Problem: I clean installed the Mac OS, and my mail/settings/address book disappeared! What happened?

Solution:

When you clean install an operating system, it creates an entirely new System Folder on the hard drive, renaming the old system folder to "Previous System Folder".

You can move the Eudora Folder contained within the Previous System folder into the existing System Folder (replacing a new Eudora Folder that has been created), and your settings and mailboxes will then function as normal.