Vista - Messages wrongly displayed as being unread

I've set up an IMAP account using Windows Mail. After getting new messages,
reading them and seeing them marked as read, those messages will
subsequently show up as unread after a "send/receive" action (manual or due
to the automatic schedule). I've adjusted the time to mark a message as read
in Tools>Options>Read(tab), but this doesn't solve the problem.

Any ideas on what is going wrong, or suggestions on how to fix this problem?