Post holder (8)
(Other definitions for envelope that I've seen before include "jacket" , "Even pole, strangely, can contain a letter" , "Postal wrapper" , "Outer covering" , "Item of mail" .)
(Other definitions for pillar box that I've seen before include "Where to put post" , "Letter recepatacle" , "Red street furniture" , "Mail receptacle" , "Cylindrical letter collector" .)