I have another hunch. Time to speak to the user...
He is now using a different client to access his mail and the problem has gone away. I have asked him to try IMP again as I wish to confirm my hunch.
Evidence suggests that he was away the day we migrated from uw-imap to Dovecot but left himself logged in to IMP. He did not return until Thursday, at which point he resumed using IMP. I suspect that he did not actually start a new session, he just revived the existing one. My hunch is that IMP became very confused when we switched IMAP servers, and fell back to re-reading the entire mailbox each time it connected to the server. If I am correct, then we should see normal usage when he next accesses IMP with a fresh session.
If I can confirm this, then I don't think there is much that can be done to "fix" the problem in either IMP or Dovecot as it is a very unique case that will rarely be encountered. And we did warn people to close their mail before the maintenance........
Thanks everyone!
-- Chris O'Regan chris@encs.concordia.ca Senior Unix Systems Administrator, Academic IT Services Faculty of Engineering and Computer Science Concordia University, Montreal, Canada