It's obviously an illegal practice. But if you want to read the US Postal Service's statement on it, refer to the Domestic Mail Manual Section 604, Sections 8.2.1 and 8.3.1 which are quoted in part below:
Quote:
8.2.1 Handling Mail With Omitted Postage
Matter of any class, including that for which extra services are indicated, received at either the office of mailing or office of address without postage, is endorsed "Returned for Postage" and is returned to the sender without an attempt at delivery. If a mail recipient attempts to return or forward the mail without paying additional postage when required, the mail is endorsed "Returned for Postage" and returned to the recipient. If no return address is shown, or the delivery and return addresses are identical, or the delivery and return addresses are different but for the same person or organization, the piece is treated as dead mail.
And...perhaps more importantly:
Quote:
8.3.1 Penalty
Whoever knowingly and willfully deposits any mailable matter (such as statements of account, circulars, sale bills, or other like matter) on which no postage is paid, in any letterbox established, approved, or accepted by the Postal Service for the receipt or delivery of mail matter on any route, with intent to avoid payment of lawful postage thereon, shall for each such offense be fined not more than $5,000.00 per individual or $10,000.00 per organization (18 USC 1725, 3571).
http://pe.usps.com/text/dmm300/604.htm