Once created on a smartphone, the letter is then printed out on standard white paper and mailed on the user's behalf, app developer Propeller Mobile said.
The app can help "digital natives" bridge the generational gap with parents and grandparents, the company said.
"Mr. Postman lets time-poor people send real paper letters direct from their smartphones and tablets," Propeller Mobile's Martin Loat told The Daily Telegraph.
"The app is proving really popular with consumers who want to cut though the noise of today's digital world with 'old school' paper letter correspondence and lots of people have been using it to send messages to parents and grandparents -- we've even had a few children using the app to send present wish lists to Father Christmas," he said.
Mr. Postman also has template letters for corresponding with businesses, he said, and users receive a message when a letter enters the United Kingdom's postal system.
Mr. Postman is available as a free download from the Apple iTunes store.