Made me look and found it:
http://pe.usps.gov/text/dmm/D041.htm
Quote:
1.3Use for Mail
Except under 2.11, the receptacles described in 1.1 may be used only for matter bearing postage. Other than as permitted by 2.10 or 2.11, no part of a mail receptacle may be used to deliver any matter not bearing postage, including items or matter placed upon, supported by, attached to, hung from, or inserted into a mail receptacle. Any mailable matter not bearing postage and found as described above is subject to the same postage as would be paid if it were carried by mail.
http://caselaw.lp.findlaw.com/cgi-bi...l=453&page=136
Quote:
Section 1725 does not unconstitutionally abridge appellees' First Amendment rights, inasmuch as neither the enactment nor the enforcement of 1725 is geared in any way to the content of the message sought to be placed in the letterbox. Pp. 120-134.
(a) When a letterbox is designated an "authorized depository" of the mail by the Postal Service, it becomes an essential part of the nationwide system for the delivery and receipt of mail. In effect, the postal customer, although he pays for the physical components of the "authorized depository," agrees to abide by the Postal Service's regulations in exchange for the Postal Service agreeing to deliver and pick up his mail. A letterbox, once designated an "authorized depository," does not at the same time transform itself into a "public forum" of some limited nature to which the First Amendment guarantees access to all comers. Just because it may be somewhat more efficient for appellees to place their messages in letterboxes does not mean that there is a First Amendment right to do so. The First Amendment does not guarantee access to property simply because it is owned or controlled by the Government. Pp. 126-131. [453 U.S. 114, 115]
Quote:
"Whoever knowingly and willfully deposits any mailable matter such as statements of accounts, circulars, sale bills, or other like matter, on which no postage has been paid, in any letter box established, approved, or accepted by the Postal Service for the receipt or delivery of mail matter on any mail route with intent to avoid payment of lawful postage thereon, shall for each such offense be fined not more than $300."