Amber McReynolds and Charles Stewart oped in The Hill:

Vote fraud in the United States is exceedingly rare, with mailed ballots and otherwise. Over the past 20 years, about 250 million votes have been cast by a mail ballot nationally. The Heritage Foundation maintains an online database of election fraud cases in the United States and reports that there have been just over 1,200 cases of vote fraud of all forms, resulting in 1,100 criminal convictions, over the past 20 years. Of these, 204 involved the fraudulent use of absentee ballots; 143 resulted in criminal convictions.

Let’s put that data in perspective.

One hundred forty-three cases of fraud using mailed ballots over the course of 20 years comes out to seven to eight cases per year, nationally. It also means that across the 50 states, there has been an average of three cases per state over the 20-year span. That is just one case per state every six or seven years. We are talking about an occurrence that translates to about 0.00006 percent of total votes cast. 

Oregon is the state that started mailing ballots to all voters in 2000 and has worked diligently to put in place stringent security measures, as well as strict punishments for those who would tamper with a mailed ballot. For that state, the following numbers apply: With well over 50 million ballots cast, there have been only two fraud cases verifiable enough to result in convictions for mail-ballot fraud in 20 years. That is 0.000004 percent — about five times less likely than getting hit by lightning in the United States.