Tonsho - The Traveling Samurai and Sending Long Documents

It was a peaceful time, and we can imagine the samurai at home most of the time — near a town's castle, where the Japanese warriors were required to live.
What, though, about the traveling samurai? The edo period rulers had thought of that, too, and provided special posts, the tonsho, it seems.
With that in mind, what email service could Tonsho provide? Is it a motel for traveling documents or battling spam?
›› Tonsho lets you send big files (up to 100 MB) by attaching them to emails as usual no matter the email size supported by the recipient; challenging senders to one POP account to prove they're not spammers, Tonsho can filter junk mail, too.