I received these letters months ago, sent by Phillip through the United Nations postal system. Both sets of stamps promote the 17 Sustainable Development Goals adopted at the 2015 UN Summit.
The stamps above were issued on 24 October 2016, and sent from New York (the USA). You can see the whole set here. Have you noticed that pretty postmark?
The letter below arrived from Vienna. The Smurfs stamps were launched to celebrate the International Day of Happiness on 20 March 2017. They want "to encourage people to take action to protect the planet and promote a peaceful and just world", with the slogan Even a small Smurf can make a big difference!