37 CFR § 2.197 - Certificate of mailing.
(a) The filing date of correspondence submitted under this section is the date of deposit with the USPS if the correspondence:
(1) Is addressed as set out in § 2.190 and deposited with the USPS with sufficient postage as first-class mail; and
(2) Includes a certificate of mailing for each piece of correspondence that:
(i) Attests to the mailing and the address used;
(ii) Includes the name of the document and the application serial number or USPTO reference number, if assigned, or registration number to which the document pertains;
(iii) Is signed separately from any signature for the correspondence by a person who has a reasonable basis to expect that the correspondence would be mailed on the date indicated; and
(iv) Sets forth the date of deposit with the USPS.
(b) If correspondence is mailed in accordance with paragraph (a) of this section, but not received by the Office, the party who mailed such correspondence may file a petition to the Director under § 2.146(a)(2) to consider such correspondence filed in the Office on the date of deposit with the USPS. The petition must:
(1) Be filed within two months after the date of mailing;
(2) Include a copy of the previously mailed correspondence and certificate; and
(3) Include a verified statement attesting to the facts of the original mailing.