Wiktionary
carry the mail
vb. (context chiefly US idiomatic English) To work diligently; to serve as the principal performer of a demanding task or set of tasks.
Usage examples of "carry the mail".
In 1886 Moses Pendergrass put in a bid for the contract to carry the mail on the route from Knob Lick to Libertyville and Coffman, thirty miles a day, from July 1, 1887, for one years.