Is there a way to protect email using passcode in iPhone?
You will have to jailbreak your iPhone to password-protect the Mail app. Sorry, there's no other way. :-)

I ran across this article, which has three apps for password protecting iOS apps.


Lockdown Pro
FolderLock
Locktopus


All are paid apps.

This article mentions an app called iAppLocker. It is free. Hope this helps you!