To mark it as unread after having abieto follow these simple steps:
Sign 1.-mail application from the home screen of your iPhone or iPad.
2.-Choose the "All" in the "Inboxes"
3.-Click "Edit" at the top right of the screen.
4.-mark the message or messages you want to mark as unread
5.-Then go to the bottom right corner and click "Flag"
6.-Finally, click on "Mark as unread".
Thus, the email messages you have marked, appear as received.
iMore.com