Explainer: Fraud is rare in U.S. mail-in voting. Here are the methods that prevent it https://www.reuters.com/article/us-usa-election-vote-by-mail-explainer-idUSKBN2482SA

Ballots aren't counted if they aren't printed on the proper type of paper and don't include specific technical markings.

States also require voters to sign the outside of their envelope, which they match to a signature on file.

Those envelopes are typically opened by a different group of workers than those who scan the ballots. Outside observers are allowed to monitor the process to ensure voter privacy.

The conservative Heritage Foundation, which has warned of the risks of mail voting, found 14 cases of attempted mail fraud out of roughly 15.5 million ballots cast in Oregon since that state started conducting elections by mail in 1998.

The most prominent cases of mail fraud have involved campaigns, not voters. North Carolina invalidated the results of a 2018 congressional election after state officials found that a Republican campaign operative had orchestrated a ballot fraud scheme.

Mail ballots can pose additional barriers to those who don't speak English or have disabilities, and delivery can be problematic on Native American reservations, where residents sometimes don't have street addresses.