Here's the explanation: http://en.wikipedia.org/wiki/Post_horn http://en.wikipedia.org/wiki/Post_horn The post horn...used to signal the arrival or departure

Here's the explanation: http://en.wikipedia.org/wiki/Post_horn http://en.wikipedia.org/wiki/Post_horn The post horn...used to signal the arrival or departure of a post rider or mail coach. It was used especially by postilions of the 18th and 19th centuries...The instrument is still used as the logo of national post services in many countries. The logo I posted was for the German mail service, but you'll see variations of that image used to mean 'post' all over Europe, and in Australia.