Set a password on the phone and you are fine (as you have done_, there is no way to get it to prompt you for a password each time, that would make no sense as it is constantly polling for new email so it would be asking you for the password every 5 seconds or whatever. The Microsoft account on your phone is tied directly to your outlook.com/hotmail account so they are the same thing.
So basically, set a passcode on the phone that is secure, this is your only way to security.
Zen 8000 Pro