Last week I talked to the Republicans who run the elections offices in Washington and Utah, two vote by mail states, about how the systems work and how they prevent fraud. It’s not just doable, it’s done all the time. https://www.washingtonpost.com/politics/paloma/the-trailer/2020/04/02/the-trailer-the-fear-and-politics-around-expanding-voting-by-mail/5e84980e602ff10d49ada414/

tl; dr version: Mail ballots need signatures. If signature doesn’t match, voter is contacted and given time to fix it. If voter can’t be contacted, ballot isn’t counted.

So when someone suggests, for example, that ballots could be filed in en masse for dead voters still on the rolls, they either don’t know how this works or are pretending not to.