The Mailbox is, as the name suggests, a mailbox. It is a static world object on Earth that appears in ToeJam & Earl and ToeJam & Earl: Back in the Groove.
It appears as a metal box with a slit and painted blue. Standing next to the mailbox will give the option of using bucks to buy from a selection of random presents from it (this may suggest that a postal worker or mail order worker is actually inside). These could be either good or bad presents, and if it is of a type of present not yet opened or identified by the Wiseman, they will show up as a series of question marks.
A certain species of monster will hide in disguise as a mailbox, awaiting for an unsuspecting mailbox user to approach. They can normally be identified from a distance by the eyes that occasionally peep out from inside.
If you visit every Backer character in ToeJam & Earl: Back in the Groove, then you will unlock the Special Mailbox present. When opened, this spawns a unique variant of the Mailbox with yellow fur and a purple hat. All mail order items will be $1 from this mail box.