As a child I used to collect stamps. It was neat to have little bits of countries from around the world. It was also neat to learn about each country basted on what they printed on their stamps. Be it their leaders, wildlife, heros or great national accomplishments. What goes on a stamp is a window into what a country (or at least their government) cherishes.
It is rare that I ever receive stamps anymore. So it was a great treat to receive a letter recently (from the Western Wilderness Committee) that had a real live stamp on it. There were two in fact. One for L. J. Papineau and another for Olympic champion Percy Williams.
There is just something a bit more authentic about a letter with a stamp. Something you don't get with the pre-printed on machine stamped letters. With a real stamp it immediately doesn't feel like junk mail. I'm sure someone has tried this but it would be interesting to see if a DM campaign would have a higher response rate if it had a real stamp (and at that one with content somehow relevant to the offer or company) rather than the usual personality-void mechanic stamp.