There is no evidence to back claims that mail-in voting is particularly susceptible to fraud. Mail ballots must be signed by the voter, and the signature is compared to the signature put on file with the state when the voter registered.
In the past, absentee ballots tended to be cast disproportionately for Republican candidates, and the Republican Party made special efforts to mobilize its supporters to vote absentee. But more recently, as mail-in voting was popularized, particularly in the western states like Colorado, Oregon, Washington, California and Arizona, and became available to anyone, not just those with a health or occupational issue, the partisan advantage has been reversed. In many 2018 congressional and statewide elections, Republican candidates led in the same-day voting at precincts, but were defeated once mail-in ballots were added to the counts.
https://www.wsws.org/en/articles/2020/05/27/elec-m27.html
