I'm responding to my own post to say I've found the problem. The "View Unread" option was set for the Spam folder - I certainly didn't set this as I had no idea it existed. All my other folders have "View All" set.
Geoff Roynon -- Using PowerMail 4.2.1 (SpamSieve 2.0.2) on a G4/450, 512 MB RAM, under MacOSX 10.2.6 (Classic 9.2.1)
