If you voted by mail in Massachusetts, you can check whether the state has received your ballot at https://www.sec.state.ma.us/wheredoivotema/track/trackmyballot.aspx

Mine shows as "accepted" and that it arrived on August 17 (they sent it to me on the 10th).

(I found this while wondering how to use the in-person early voting if I had asked for and been sent a postal ballot.)