FWIW, the series is one of my favourites and I recently reread. I don't know where it's set out but the Messenger's principle job is to organise the communcations relays, mail coaches, etc.

A lot of communication of importance is carried by people other than the narrator, but he carries the stuff that's urgent, by dint of being the fastest person alive.

However, while I know it's set out in at least one of the books, there are four of them and I don't know where all the details are set out.