Mail message viewer displays wrong message

Question

Here's a strange one, started a couple of days ago: for most new messages, when I select the message in the upper pane, the message displayed in the viewing pane is not the one I selected.
For example: a new message comes in from Joe, I click on it, but what I see is the text of a message from Dave that's three years old (and of course still sitting in my inbox---I can go find it and look at that one too, and it still displays normally). The messages I'm seeing in error are old messages in my Inbox, coming up in order, starting around message 500 (there are almost 2000 messages in my inbox). I can't read the content of the new message using Mail.
My Mail is set up to leave everything on the server (it's an IMAP account), which is lucky since I can then access the mail by other means. For example, the Mail program on my laptop still functions normally, so I've been reading mail that way. In fact my two computers are both running OS 10.6.4, with identical Inboxes. Could there be some difference between how my relatively new iMac and my 1.5-yr old MacBookPro handle mailboxes?
Other ideas?

Answer 1

this solved the problem for me:
http://discussions.apple.com/thread.jspa?messageID=12184634&#12184634
in short: in Mail select the broken mailbox, then go to the menuitem Mailbox -> Rebuild
Hope this helps.