There is a very good article from Snopes.com on May 26, 2020. They rate the claim of massive mail-in voter fraud as "Mostly False".
https://www.snopes.com/fact-check/mail-in-ballot-voter-fraud/
The Heritage Foundation finds 204 cases of Fraudulent Use of Absentee Ballots in all 50 states over the period from 1992 - 2016, more than twenty years of data. That is out of more than about 1 BILLION votes cast. See https://www.heritage.org/voterfraud/search?combine=&state=All&year=&case_type=All&fraud_type=24489 for the database. The Heritage Foundation defines FRAUDULENT USE OF ABSENTEE BALLOTS as "Requesting absentee ballots and voting without the knowledge of the actual voter; or obtaining the absentee ballot from a voter and either filling it in directly and forging the voter’s signature or illegally telling the voter who to vote for."
The Brookings Institution has an article on its web site: Low rates of fraud in vote-by-mail states show the benefits outweigh the risks by Elaine Kamarck and Christine Stenglein from Tuesday, June 2, 2020 that examines the Heritage Foundation database used in the report noted above. The Brookings report has an interesting table on the number of cases of mail-in voter fraud in the five states (Colorado, Oregon, Utah, Hawaii, and Washington) that used all mail-in voting before 2018. Their total from the Heritage Foundation database of mail-in voter fraud cases is at
https://www.brookings.edu/blog/fixgov/2020/06/02/low-rates-of-fraud-in-vote-by-mail-states-show-the-benefits-outweigh-the-risks/ Their conclusion is that while mail-in voter fraud does occur, the number of cases is infinitesimal.
There is also a good article from the MIT Election Data + Science Lab at https://electionlab.mit.edu/research/voting-mail-and-absentee-voting that discusses the history of absentee and mail-in voting. Note that the director of the MIT Election Data + Science Lab is Charles Stewart III, Ph.D., one of the authors of "The Hill" article below.
There is an article at "The Hill" web site (not known for its liberal leanings): Let's put the vote-by-mail 'fraud' myth to rest by Amber McReynolds and Charles Stewart III, from April 28, 2020. See
https://thehill.com/opinion/campaign/494189-lets-put-the-vote-by-mail-fraud-myth-to-rest This article also does an analysis of the Heritage Foundation database and comes to the conclusion that in the case of widespread voter fraud "This is simply not true."
Finally, here is a link to the Federal Election Commission's Twitter accumulation by one of the commission's members on mail-in voter fraud from May 27, 2020: https://www.fec.gov/resources/cms-content/documents/2020-05-27-ELW-Facts-About-Voting-by-Mail.pdf
In short, voter fraud of all types does exist, but is exceedingly rare and has not been shown to change the results of any election. Data is your friend.
