37 C.F.R. § 2.119 : Patents, Trademarks, and Copyrights — UNITED STATES PATENT AND TRADEMARK OFFICE, DEPARTMENT OF COMMERCE — GENERAL — RULES OF PRACTICE IN TRADEMARK CASES —PROCEDURE IN INTER PARTES PROCEEDINGS — Service and signing

(a) Except for the notice of opposition or the petition to cancel, every submission filed in the Office in inter partes cases, including notices of appeal to the courts, must be served upon the other party or parties. Proof of such service must be made before the submission will be considered by the Office. A statement signed by the attorney or other authorized representative, attached to or appearing on the original submission when filed, clearly stating the date and manner in which service was made will be accepted as prima facie proof of service.
(b) Service of submissions filed with the Board and any other papers served on a party not required to be filed with the Board, must be on the attorney or other authorized representative of the party if there be such or on the party if there is no attorney or other authorized representative, and must be made by email, unless otherwise stipulated, or if the serving party can show by written explanation accompanying the submission or paper, or in a subsequent amended certificate of service, that service by email was attempted but could not be made due to technical problems or extraordinary circumstances, then service may be made in any of the following ways:
(1) By delivering a copy of the submission or paper to the person served;
(2) By leaving a copy at the usual place of business of the person served, with someone in the person's employment;
(3) When the person served has no usual place of business, by leaving a copy at the person's residence, with some person of suitable age and discretion who resides there;\
(4) Transmission by the Priority Mail Express ® Post Office to Addressee service of the United States Postal Service or by first-class mail, which may also be certified or registered;
(5) Transmission by overnight courier; or
(6) Other forms of electronic transmission.
(c) When service is made by first-class mail, Priority Mail Express ®, or overnight courier, the date of mailing or of delivery to the overnight courier will be considered the date of service.
(d) If a party to an inter partes proceeding is not domiciled in the United States and is not represented by an attorney or other authorized representative located in the United States, none of the parties to the proceeding is eligible to use the service option under paragraph (b)(4) of this section. The party not domiciled in the United States may designate by submission filed in the Office the name and address of a person residing in the United States on whom may be served notices or process in the proceeding. If the party has appointed a domestic representative, official communications of the Office will be addressed to the domestic representative unless the proceeding is being prosecuted by an attorney at law or other qualified person duly authorized under §11.14(c) of this chapter. If the party has not appointed a domestic representative and the proceeding is not being prosecuted by an attorney at law or other qualified person, the Office will send correspondence directly to the party, unless the party designates in writing another address to which correspondence is to be sent. The mere designation of a domestic representative does not authorize the person designated to prosecute the proceeding unless qualified under §11.14(a) of this chapter, or qualified under §11.14(b) of this chapter and authorized under §2.17(f).
(e) Every submission filed in an inter partes proceeding, and every request for an extension of time to file an opposition, must be signed by the party filing it, or by the party's attorney or other authorized representative, but an unsigned submission will not be refused consideration if a signed copy is submitted to the Office within the time limit set in the notification of this defect by the Office.

37 C.F.R. §2.119

37 FR 7606, 4/18/1972, as amended at 41 FR 760, 1/5/1976; 54 FR 34898, 8/22/1989; 54 FR 38041, 9/14/1989; 63 FR 48097, 9/9/1998; 67 FR 79523, 12/30/2002; 72 FR 42259, 8/1/2007; 73 FR 47686, 8/14/2008; 74 FR 54909, 10/26/2009; 79 FR 63039, 10/22/2014; 81 FR 69975, 1/14/2017