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.