Mail matter
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
This term includes letters, packets, etc , received for transmission, and to be transmitted by post to the person to whom such matter is directed.
U. S. v. Huggett (C. C) 40 Fed. 641; U. S. v. Rapp (G. a) 30 Fed. 820.