Explanation: 'Mail' is a term for 'Chain mail', a type of armor widely used in the Roman Empire (they adopted it from the Gauls, and sometimes called it lorica gallica - 'Gallic armor' - for that reason).
Mail also refers to the good ol' post! The Roman Empire, starting with the Emperor Augustus, established an imperial post system inspired by and modeled after Persian royal post couriers - the Cursus Publicus. While the system was for government business, and not open to public use, there was some amount of commensalistic benefit from the infrastructure installed - you won't stay at a mansione for free as a private citizen, but it is quite helpfully along just the route any non-government courier would take!