Certainly, there are thousands of Adolf Hitler postcards out there, including perhaps the creepiest Christmas postcard ever, but these were commercially produced and clearly do not qualify as mail art.
However, during his time in Vienna (1908-1913) the young artist made a meager living by painting watercolors and selling them to tourists. Some histories say he painted postcards, others say he tinted postcards, and still others say he copied scenes from postcards. Perhaps all of these are true. He did produce hundreds of works, as many as three a day, many of which survive.
It is safe to say that he did create individual works of art as postcards, meant to be posted. But there is no record of Hitler himself ever posting them. And since the posting, the journey through the mails, and the receiving of the work are all a part of the mail artist’s process, I do not believe we can credit Hitler with being an early mail artist. He did not follow through. Close, but no cigar.