running Mt. Lion on an air but I've also seen it happen on my other machine with is a mac pro running Mavericks. I'll be looking at email and see that something I've been waiting for has arrived. I go to click on it and it disappears. I may even see the message for a split second but it disappears before my eyes. I do a search for anything from that sender and it is completely gone. It's not in the trash - it is gone. Next time I get an email, it pops up along with the new email. I've figured out the work around is to send myself an email but it's really irritating. What's the deal?