What is mailman?

Some articles on mailman:

List Of Aria Episodes - Episodes - Aria The Natural (season 2)
... Mailman borrows Akari and her gondola for a day so he can complete his rounds while his gondola is repaired ... Mailman explains that Neo-Venezians prefer the inconvience of mail because a letter, being not as quickly written as e-mail, can carry the sender's heart, and so can become a treasure, and furthermore it ... Mailman tells her that she has been painted with "the color of Neo-Venezia" ...