I have just listened to the first of a series of 15 minute programmes on the history of the Post Office, revealing how it began in the 16th century purely to convey the King's business, hence its title today - The Royal Mail. It became an instrument of state control, intercepting letters to foil treasonous plots, before evolving into the network we know today. Unfortunately, the first item in the series, broadcast today on Radio 4 at 1.45 pm, is not available on iPlayer (unless Tony can find it!) but you can listen in to subsequent programmes every day this week. There might be some interesting information about interception in Nelson's day. Nelson certainly didn't trust the post not to intercept and open letters; hence his preference for entrusting intimate letters to the care of a friend for safe conduct.
http://www.bbc.co.uk/programmes/b017vp92