Mail-in ballots are exceedingly easy to fake.
For example, in Buddy Cianci's autobiography, Politics and Pasta: How I Prosecuted Mobsters, Rebuilt a Dying City, Advised a President, Dined with Sinatra, Spent Five Years in a Federally Funded Gated Community, and Lived to Tell the Tale, Buddy described how in two mayoral elections he won, a certain Providence city councilor conveniently "found" mailed-in ballots that helped sway the election in Buddy's favor. This particular city councilor had done this "finding" on previous occasions and represented a part of South Providence. South Providence is a primarily-minority area of the city and suffers from urban blight to this day.