how I mark mail as unread iPhone

I set up my gmail account using microsoft exchange. When I open a message, sometimes I want to mark it as unread to remind me to read it again later. The iphone user guide gives the instruction open the message and tap mark as unread. Seems simple enough, but I cant find where on the screen I am suposed to tap?? I have a swirly arrow, a file folder, a trash can, an arrow pointing left, and a square with a pencil in it in the bottom row of icons...
iPhone4 iOS 4