A mailbox out in the Wasteland
Stanley Express Mailboxes are the Stanley Express package delivery boxes featured in RAGE. The mailboxes are quest related world objects.
After completing the quest "Wellspring Bound" a character named Stanley in Wellspring will offer Nicholas Raine three mail delivery missions — "Settler Postal", "Bandit Postal", and the "Northern Express", — depending on which route of the same name the protagonist will choose. Each side quest involves driving around the Northern Wasteland to a set number of mailboxes within a certain amount of time.
The mailboxes can be found throughout the Northern Wasteland. Some boxes are clearly seen from a distance (e.g. the ones near the Hagar and Outrigger Settlements), and some might require a certain effort to be found.