Mailing Obscene Materials - Any material that satisfies this three-pronged test may be found obscene:
Whether the average person, applying contemporary adult community standards, finds that the matter, taken as a whole, appeals to prurient interests (i.e., an erotic, lascivious, abnormal, unhealthy, degrading, shameful, or morbid interest in nudity, sex, or excretion); whether the average person, applying contemporary adult community standards, finds that the matter depicts or describes sexual conduct in a patently offensive way (i.e., ultimate sexual acts, normal or perverted, actual or simulated, masturbation, excretory functions, lewd exhibition of the genitals, or sado-masochistic sexual abuse); and whether a reasonable person finds that the matter, taken as a whole, lacks serious literary, artistic, political, or scientific value.
Source: http://www.justice.gov/criminal/ceos/citizensguide/citizensguide_obscenity.html
Whether the average person, applying contemporary adult community standards, finds that the matter, taken as a whole, appeals to prurient interests (i.e., an erotic, lascivious, abnormal, unhealthy, degrading, shameful, or morbid interest in nudity, sex, or excretion); whether the average person, applying contemporary adult community standards, finds that the matter depicts or describes sexual conduct in a patently offensive way (i.e., ultimate sexual acts, normal or perverted, actual or simulated, masturbation, excretory functions, lewd exhibition of the genitals, or sado-masochistic sexual abuse); and whether a reasonable person finds that the matter, taken as a whole, lacks serious literary, artistic, political, or scientific value.
Source: http://www.justice.gov/criminal/ceos/citizensguide/citizensguide_obscenity.html