There are two kinds of mail balloting systems. Some states have what are called universal “vote by mail” in which states mail ballots to all voters. In most states, however, vote by mail is through absentee balloting in which the voter must request an absentee ballot. In 2016, nearly one-quarter of U.S. votes (33 million) were cast by either universal mail or absentee ballots.

As shown on the map below from the National Council of State Legislatures, 34 states plus the District of Columbia now allow voters in the weeks before an election to request absentee ballots. Another 11 states have made it easier to request absentee ballots for primary elections taking place this year, in large part due to concern over the coronavirus.

BI https://www.brookings.edu/policy2020/votervital/how-does-vote-by-mail-work-and-does-it-increase-election-fraud/