The built in Mail app should show new emails only.
There appeared an issue for quite a few users when updating to iOS 7, though: they suddenly discovered hundreds or thousands of unread emails in their inbox. In this case simply open the Inbox of the email account, tap "Edit" (top), then you'll see "Mark All" at the bottom left. Tap it. You'll have to decide between "Flag" and "Mark as Unread" now. Select the latter. Then again: Edit, Mark All, but this time "Mark as read" will appear, which you should tap now.
If all the emails in that Inbox are unread, you'll see the second version when you do this the first time, so you don't need to go on.
Hope that helps.
