From the The Mail Coach Service. The Royal Mail: Postal Heritage Trust (2005):

“The postal delivery service in Britain had existed in the same form for about 150 years – from its introduction in 1635, mounted carriers had ridden between “posts” where the postmaster would remove the letters for the local area before handing the remaining letters and any additions to the next rider. The riders were frequent targets for robbers, and the system was inefficient.”