From Robert Woodhead:
Starting with Eudora 5.2, it is now possible to check an incoming email to see if it matches against the contents of an address book. This is very useful if you want to prioritize your inbox, and if you are using anti-spam software, it can prevent the dreaded "false positive".
BoxSweeper makes this feature easier to use. You simply dump all your saved email into BoxSweeper, and it scans all the emails, constructs a list of all the email addresses it can find, and tells Eudora to add them to an address book. Now you can, in a single filter operation, mark emails that have come from people you've previously interacted with.