The definition of mailing it in is to perform a given task, duty or activity with little or no attention, effort, or interest in actually doing something, ie, doing it in a perfunctory manner.   

For example, a key player on the gridiron, the team star running back, “seems to be mailing it in this afternoon.”

This is distinguished from a reporter excitedly calling in a news scoop to the Editor.   

Don’t go through the motions: Deliver!