It is so important, as we head into the final three months before November 3, to both acknowledge and understand the critical difference between the reasonable concerns related to mail-in voting and the partisan talking points about a "rigged" election.
This piece from Reason does a nice job of describing the distinction. Worth a read, and worth circulating as a rebuttal to folks who believe the conspiracy theories.
Contrary to Trump's often repeated claims about "totally rigged" voting by mail, however, there is no evidence that the problem is widespread or that it has affected recent election outcomes. "Election fraud in the United States is very rare, but the most common type of such fraud in the United States involves absentee ballots," Rick Hasen, an election law expert at the University of California, Irvine, law school, told the Times in April. "Sensible rules for handling of absentee ballots make sense, not only to minimize the risk of ballot tampering but to ensure that voters cast valid ballots."