The Tibian Postmaster's Guild takes care of all mail delivery in Tibia. They also maintain the mailboxes and Royal Mailboxes deep in the wilderness.
Kevin Postner is the Head of the guild, sitting upstairs in the Postal Service Headquarters located here. Several Postman NPCs take part of the quest, and all the NPC members of the guild respect the discounts to other guild members. Their presence can be seen on every major city and even on such uncivilized places as Roshamuul and Krailos. Its members are mainly human, but there are dwarves, a gnome, a lizard, a golem and even a demonic entity and a rock among their ranks.
On the majority of cases, if you see a 
sign on the ground or a 
sign hanging next to the entrance of a building, you'll find a postman inside.
Some of the members, which are not players nor NPCs, are on charge of the field job. They are inferred to be strong adventurers themselves, strong enough to be able to retrieve the parcels and mail deliveries from the Royal Mailboxes and deliver them to the destinataries' Depots. Some of these mailboxes are in hard-to-access areas as The city of the Necromancers, of the minotaurs, of the Orcs, of the barbarians, the hideout of the pirates, the vampire castle, an evil castle with evil entities, inside the capital of the dragon empire, an underground island, at the middle of the seabed, in the depths of a cultist base and even a Hydra-infestated island. These locations show the great presence of the Tibian Postmaster Guild all around Tibia, and the untold work of delivery postmen and postwomen.
Finally the guild has also an agreement with Ship Captains to give discounts to higher ranked members, see this table for more info.
For more information about applying to be a member and receiving its benefits, see this page.