For all practical purposes? Nothing.
You are wrong, and repeating a typical Democrat talking point.
Absentee voting, and many posters have noted, is a process the voter initiates by asking for a ballot. That means that the voter is in control, and can take steps to ensure that they get the ballot, vote, and then return the ballot. Election officials only send ballots to legitimate, registered voters, and they normally check that the received ballots are from the voter who requested them.
By contrast vote by mail states send out ballots to anyone on a list of registered voters, to whatever address they had the last time they voted. The only security is some kind of signature check when the ballots are received back in the mail.