https://www.sec.state.ma.us/ele/eleev/early-voting-faq.htm https://www.sec.state.ma.us/ele/eleev/early-voting-faq.htm has the official FAQs from the Massachuset

https://www.sec.state.ma.us/ele/eleev/early-voting-faq.htm https://www.sec.state.ma.us/ele/eleev/early-voting-faq.htm has the official FAQs from the Massachusetts Secretary of the Commonwealth and says you can look up whether your ballot was accepted. It's not totally clear what reasons they might use to reject a ballot though.