Mail app - prompt for password ?

First thing that has annoyed me with the Mail app is it doesn't have a 'prompt for password' option, this means if someone gets access to my iPod they can get my email by just pressing the Mail icon.

Anyone know if you can get it to prompt for a password each time you want to check email ?

BTW I have got a lock on the iPod itself but its inconvenient if I don't want to check mail which I don't most times I use my iPod