After running JDiskReport, I discovered an enormous wad of mail waiting in mailing lists I have not read for some time. I like the fact that Apple Mail provides the Activity Viewer window, in which you can track what the application is doing—especially useful when it otherwise looks like it has stalled. Here is the result of trying to delete 10,000 unread mails using IMAP over a pretty slow uplink:
It has been churning away for at least 20 minutes. I'm sure it will get there eventually, but I really wish I had done this server-side using Mutt.