All early voting ballots submitted by mail, delivered in person to the office of the city or town clerk, returned to a secured municipal drop box for the city or town where the voter is registered or returned electronically pursuant to the accommodations granted to a voter by reason of disability under paragraph (4) shall be received by the city or town clerk before the hour fixed for closing the polls on the day of a preliminary, primary or election; provided, however, that an early voting ballot mailed on or before the day of a biennial state election shall be accepted until 5 p.m. on the third day after the election and shall be processed in accordance with section 95. A postmark, if legible, shall be evidence of the time of mailing.
All envelopes referred to in this section shall be retained with the ballots cast at the preliminary, primary or election and shall be preserved and destroyed in the manner provided by law for the retention, preservation or destruction of official ballots.
Mass. Gen. Laws ch. 54, § 25B