The Mailbox Gremlin was one of the many gremlins that invaded the town of Kingston Falls. It is an unseen but heard gremlin that was hiding in a mailbox to mess with people who tried to send their mail.
The Kingston Falls reverend, Father Bartlett, to comes up to the mailbox and puts his letters into it, but they are sent back out by the gremlin who makes disgusted sounds much his surprise. When another man, Mr. Anderson, then comes up next to the reverend and puts his hand into the mailbox to send his mail, the gremlin inside apparently bites Anderson's hand and doesn't want to let go.
He later joins the other gremlins inside the Kingston Falls Movie Theater later on and dies along with the other when it explodes due to Billy Peltzer putting on the gas and lighting a fire in there to finally get rid of the entire gremlin horde.