I am not a blackberry user, so I am not sure, but is there a view option set to view only unread mail? That would explain it “disappearing” after you read it. I often see this with outlook users when they claim there mail “disappears” after they read it. Changing the view to all mail instead of unread magically makes the “missing” mail re-appear. I just don’t know if this applies to blackberries too.