I guess I have not seen any factual basis to conclude that mail-in voting is problematic. I get the theoretical argument, and can imagine all sorts of USPS conspiracies to rig the vote, but the fact is we have multiple states that allow mail-in voting, where millions of voters have cast ballots by mail, and both parties have won and lost elections while watching and recounting numerous votes... and there is no indication that this process has been problematic, ever. And certainly no evidence that it is not at least as secure as the voting machines we have, while still facilitating more people voting.