18 USC 1461 is sometimes known as the
Comstock Act, named for nineteenth-century morals crusader Anthony Comstock. Originally passed by congress in 1873, there have been revisions of this legislation throughout the years, the last being in 1994.
Although this law is still on the books, several court cases have drastically altered its reach. For instance, the law does not apply to literature regarding
legal abortions, since such a regulation of speech would contravene the first amendment. Also, for a mailing to be banned under this statute, it must be deemed "obscene" by the local community standards (see
Smith v. United States). Prosecutors typically find such community standards to be pretty lax, and so are largely unwilling to undertake these kinds of pornography prosecutions (there was a brief flurry of porn prosecutions at the beginning of Ashcroft's tenure as attorney general, but that effort was not sustained).
In short, court decisions have decidedly narrowed the scope of the law, and prosecutorial reluctance to file criminal charges means that this law is rarely enforced.