Having worked at the polls as a sworn Inspector for the past 15 or so years, I can attest to the fact that everyone I've worked with at the polls takes election integrity very seriously. We are committed to a fair process that allows every registered voter to have their vote counted.
Recently, a certain president has made an effort to discredit elections that he might not win. (Don't forget in 2016, he lost several GOP state primary contests and immediately labeled them "rigged." )
While I'm aware of historical efforts to "buy" election, and rare cases of ballot tampering, elections in this country and this state (Mass.) are reliably managed by individuals who take the vow of neutrality seriously.
The effort to discredit mail-in voting has been ramping-up. The first issue is the registered voter database. Critics have cited cases where the rolls still contain thousands of names of people who have died or moved away. They say these are possible means of voter fraud. They worry that someone could send mail-in ballots for these dead and gone people. Except that's not what happens in real life. Some states actually compare signatures on mailed-in ballots against registration signatures. Most of the horror stories that are shared on social media involve some conspiracy theory, easily believed by gullible partisans. Very few actual cases have been proven.
The penalties for tampering in an election are severe. This dissuades most potential tamperer.
I believe Trump has been tampering with the election process since he was nominated and even more so since he was inaugurated. I will not be surprised when, after being soundly defeated in the coming the November election, Trump and some of his appointees are indicted for election tampering. This time he will not have a claque of enabling senators to save him.
Secret ballot. Some have raised the question of mail-in ballots not being secret. Again, in my experience poll workers are sworn to maintain secrecy. Typically, absentee ballots were aggregated as received, and not opened until voting day, when the Warden would feed the ballots into the voting machine, only examining a ballot if there was a malfunction (e.g. mis-marking of the ballot, causing it to be rejected). Sometimes a voter will have their ballot rejected by the machine. The Warden will always ask for permission to look at the ballot to determine the problem. If the voter says "No," the problem ballot is destroyed and a new blank ballot is issued. I am not sure how they do this in places where there is not a paper ballot.
Provisional ballots. These are ballots that are given to people who show-up to vote but are not on the rolls. At the discretion of the Warden, a ballot may be issued but is not tabulated until the claim of the voter can be reviewed/cross checked. (In practice, there are only handful of provisional ballots and are counted only if they could affect the outcome, say, in a recount situation)
Ballot Harvesting. I think this is a recent concern stemming from the recent changes to allow large numbers of mail-in (versus "absentee") ballots. Ballot Harvesting is when a third party (other than a relative or postal worker) collects ballots to deliver to a polling place. Massachusetts does not address the legality of this practice, but it does seem open to possible abuse, and I would support making ballot collecting or harvesting illegal.
Due to the pandemic, I will not be working the in-person polls this November, but I feel confident that the results will not be "rigged."