The default Mail app that comes on the iPhone, iPod Touch and iPad is great but it does not lets you mark all the email as read or unread. It also does not lets you delete / hide or mark all the emails. Fortunately, there is a jailbreak tweak available at Cydia that lets you do exactly that.
The tweak is called Allmail and it is compatible with both iOS 4 as well as iOS 5. The only downside is that the tweak is not free and costs 1.50 dollars. After installation, there is no icon that is added to the homescreen. However, configuration options are available in the Settings.