I have a computer, and I let my roommate and his girlfriend use it once in a while. I don't have anything scandalous in my e-mail. I am just concerned that they can access my inbox if for some reason I am not around.

For example, I have my Outlook set up so no new messages are downloaded without a password. However, all of my already received messages are still visible. I am not sure if there is any way to disable that so nothing would be seen without a password.