Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 54, § 93

All absent 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 section 91C shall be received by the city or town clerk before the hour fixed for closing the polls on the day of a primary or election; provided, however, that an absent 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.