305.02(b) Mailing Requirements
The correspondence must be properly addressed (see TMEP §305.01 for mailing addresses), and must be deposited with the U.S. Postal Service (USPS) with sufficient postage as first-class mail. 37 C.F.R. §2.197(a)(1).
Since first-class mail services of the USPS are not available in foreign countries, the certificate of mailing procedure may not be used for sending mail to the USPTO from a foreign country.