18, U.S.C., 1461, makes it a Federal crime or offense for anyone to use the United States mails to transmit obscene materials.

A person can be found guilty of that offense only if all of the following facts are proved beyond a reasonable doubt: First: That the person knowingly used the mails for the conveyance or delivery of certain articles, as charged; Second: That the person knew at the time of such mailing the general nature of the content of the matter so mailed; and Third: That the matter so mailed was "obscene" as defined by law. (Note: Willfulness not an essential element.)