Non-Mailable
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
A term applied to nil letters and parcels which are by law excluded from transportation in the United States malis, whether on account of the size of the package, the nature of its contents, its obscene character, or for other reasons.
See U. S. v. Nathan (D. C.) 61 Fed. 936.