Sometimes you just have to use common sense. Looking at other places also helps.
Just one example.
"Their lawyers made a series of allegations, including "personation" in postal voting and at polling stations and ballot paper tampering.
The Election Commissioner upheld a number of the allegations, including:
Voting fraud: ballots were double-cast or cast from false addresses"
https://www.bbc.com/news/uk-england-london-32428648
And vote harvesting, which is currently legal in California... where do you stand on that relative to mail in ballots? No room for fraud?
You're incredibly NAIVE if you think voter fraud doesn't exist, regardless of the method used for voting.