Many people ask this question including myself. I have been to The Netherlands, and asked many people the question. They mostly believe it stems from the mail, (The Mailman) which in Holland as well as other countries in Europe is called the Post. Some people say it is from the Dutch word for "seagull" but that is not the word for seagull, there is a dove called the Post Dove. Some say the name originates in Germany where in about the year 800 a couple signed as witnesses to the transfer of property. In olden days there was a marker which devided two towns (we still have them), the marker was called the "post" and so a person who lived by the marker became knows as, "John of/by the Post" or later John Post.

Posts seem to have come to America from England, Denmark, and Germany.

My father has taken a real interest in this line and has looked a several sources. All my information has come from him, but I haven't done a good job at getting copies of his sources.