Sightline Institute contends that voting by mail-in ballots is secure and safe from fraud.

If….

And that is a big if.

1. States follow “best practices” and keep their voter rolls clean.  (11 counties in California have voter registration rates exceeding 100 percent of the age-eligible citizenry according to 2019 law suit settlement  and ERIC).
2. You can’t vote more than once if signatures are verified if states follow “best practices.” (Signature matching by individuals is highly subjective).
3. It is a crime to vote more than once.  (Not all states have that law).
4. It’s a crime to try to unduly influence someone else’s vote.