The novel The Sirens of Titan by Kurt Vonnegut tells the story of a robot called Salo who comes from a faraway planet called Tralfamadore.  On this planet, machines took over a long time ago and organic life became extinct.

The Tralfamadorians sent Salo on a secret mission with a message to be delivered to the other side of the galaxy.  But his space ship broke down in 203,117 B.C. and ever since he has been stranded in our Solar System on the moon Titan, waiting for the required part to be shipped to him.

It turns out that the whole of human history has been manipulated by the Tralfamadorians in order to send encouraging messages to Salo and finally to provide him with the needed replacement.  This replacement is a piece of metal, a discarded piece of technology, that one of the characters has picked up on Mars during a senseless war (also induced by the Tralfamadorians). 

Now Salo can repair his spaceship and continue his voyage.  One of the human characters, also stranded on Titan, asks Salo, please before he departs, to reveal the message he carries.  Against his inbuilt prohibition, Salo eventually complies with his request.  

The message consists of a single dot, which in the Tralfamadorian language means “greetings”.  Salo feels so depressed about the apparent futility of his mission, that he disassembles himself.