On Kawara is a Japanese artist living in New York. Between 1968 and 1979 he created and sent several series of mail named I'm still Alive and I Got Up At. In the postcards he wrote or rubber-stamped the artist's time of getting up, the date, the place where the postcards was sent from.
The length of each series ranged from a single piece of mail to hundreds sent consecutively over a period of months.
Not exactly boring correspondence! For instance, in 1973 alone he sent postcards from twenty-eight cities.