Retrieving from the dim and distant murky past of my long term memory (used to admin 800 Eudora users), there were a couple of files that made up Eudora's mailbox database. An .MBX file and a .TOC file. The MBX is the mailbox and the TOC is the Table Of Contents. Sometimes Eudora would get it's knickers in a twist and corrupt it's TOC. Thankfully though, if you shut Eudora, delete the .TOC file and start Eudora it would rebuild the TOC for you.
Try that once you have a backup of the files though, as i'm talking info from 1994ish onwards
