K-9 Mail - Hide read messages

This seems like it should be a simple question to answer, but I think I must be rather dense today. I have began using K-9 mail with two Imap accounts on my HTC Inspire 4G. How can I hide the messages I have already read? Some of them I do not want to delete, but after I have read them, I want to hide them, until I need them again. Seems simple, but for me, not so much.