The only way you can protect that e-mail (and I am assuming it's so that no one can see it if they use your iPad) is to either put a passcode lock on your iPad - which will lock anyone out from accessing the whole iPad. Or, you can delete the e-mail account before handing the iPad to someone - you'll have to re-enter/re-load it when you get your iPad back.
That's because Apple has designed the iPad, essentially, to be a one person tool. There is no way to just lock the Mail app. Sorry.
Marilyn