We don't know that the mail-in ballot system here in California is absolutely, with 100% certainty, immune to abuse. We do have reasonably good circumstantial evidence at this point that it does not appear to increase the chance for voter fraud, and furthermore, we have reasonably good evidence, based on multiple studies conducted over many years that anyone can easily find if they care to, that there are very, very few fraudulent ballots cast in American elections. There is, however, also reasonably good evidence that American elections have a history of efforts to prevent eligible voters from casting votes at all, and that this is far and away the kind of "voter fraud" that we need to be concerned about.
As a general axiom, therefore, in American elections, campaigns that have as their goal making it more difficult for eligible voters to vote in the name of "reducing fraud" should be viewed with, well, a high degree of suspicion.
