Fact: Neither republicans nor democrats are likely to abandon their political party; therefore, this letter is intended for that group of independents who remain undecided. They know a 100% truthful politician is hard to find. A false statement could be just an unintended slip of the tongue, or it is possible truth is not of the slightest importance to the speaker.
Several studies have shown mail-in ballots are almost always legitimate; an additional safeguard this year, in California anyway, is the ballot can be tracked until it is tabulated at the election center. Claims to the contrary are intended to create confusion and reduce voter turnout.
False claims lacking the slightest iota of proof are appearing also. A fairly recent one, stating the Bidens had received money from Russia, was so questionable the author did not attach his name. Unverified claims by any person or political party should be ignored. If there is any doubt, do some research.