According to Mark Hemingway on RealClear Politics:

Between 2012 and 2018, 28.3 million mail-in ballots remain unaccounted for, according to data from the federal Election Assistance Commission. The missing ballots amount to nearly one in five of all absentee ballots and ballots mailed to voters residing in states that do elections exclusively by mail.

Ballots that were “spoiled, undeliverable, or came back for any reason” (including voting) were not counted.  These are ballots that went out and were never seen again.  Obviously, any fraud would be included in the ballots that were actually used, but that isn’t the concern, here.

This large number of ballots represents a huge, fertile field which campaigns with enough money could harvest.  If there are millions of people who have ballots on their desks that they will not use, then a politician who can find his or her supporters among them (or people willing to cast a vote as requested), that’s potentially a massive advantage.

We’ve seen evidence of this sort of activity in Rhode Island in recent elections, as campaigns have hired people to become notary publics who then go out and “assist” with mail balloting.  Most notorious was when Democrat Speaker of the House Nicholas Mattiello seemed unconcerned about his polling-place loss to Republican Steve Frias because he was so confident in his number of mail ballot supporters.

Anybody who is concerned about the possibility that people could buy votes, this should be a frightening prospect.  Yet, the political establishment in Rhode Island keeps pushing and shoving Rhode Island in the direction of more mail ballot use.