How to mark as READ all the e-mails received in iPhone?
This article on Lifehacker a few days ago shows a trick to mark all emails as read.

Quoting the article:


  Open up the Mail app and tap the Edit button.
  Select one message.
  Press and hold the Mark button at the bottom of the screen.
  While still pressing and holding the Mark button, tap the email you previously selected to unmark it.
  Release the Mark button.
  Tap the Mark as Read button.