The Mailman was seen as an introductory character in the Season Three episode, "Mr. Hankey's Christmas Classics".
Background
The Mailman is the opening narrator who parodies the mailman character Special Delivery Kluger in Rankin and Bass' Santa Claus is Comin' to Town stop motion animation Holiday special, who was modeled after and voiced by [Astaire] .
The Mailman sings the Mr. Hankey, the Christmas Poo (song) with various children of South Park in the first song of the Third Holiday special.