Section 99. All envelopes received by clerks of cities and towns after the hour fixed for closing of the polls on the day of election shall be retained by them unopened until the time set by law for the destruction of ballots cast, at which time the envelopes shall likewise be destroyed, unopened and unexamined; provided, however, that such envelopes received within ten days following a state or city final election and mailed on or before the day of election, from a location outside the United States, shall be processed in accordance with the second paragraph of section ninety-five. A postmark, if legible, shall be evidence of the time of mailing.