Ted Mack and the Original Amateur Hour. 1948 to 1980
A direct descendant of radio's "Major Bowes Original Amateur Hour" (1934-1946), hosted by Major Edward Bowes until his death. After a one-year hiatus, Ted Mack, who had directed Bowes' auditions, revived the show (which lasted into 1952) and brought the concept to the DuMont Television Network. The at-home audience voted by postcard for the favorite, winning performer(s) each week.
Like others here, I only remember having heard the name of the show. Seems like there were other shows that included viewer postcards, but can't remember which ones. I do recall that some of them were for game or sweepstakes entries, and some were viewer mail.
"Music can change the world because it can change people." - Bono