"Arrived safely, will write more later." "Having a wonderful time, wish you were here." The briefest of messages, displayed for all the world to see, is scrawled across the back of a picture postcard of a placid lake or a small-town main street. Historic black-and-white images, gorgeous linen postcards, and contemporary chromes all tell stories of the state and its visitors over the decades. In Minnesota in the Mail, curator and postcard collector...