From the Document: "The 2020 primaries saw a dramatic shift in the use of mail and absentee ballots. In Kentucky, absentee ballots usually represent less than 2% of votes in Kentucky. The 2020 primary saw record turnout of an estimated 1.1 million voters with an estimated 75% casting their ballot absentee. Similarly, 1.75 million mail ballots were requested in New York, 11 times more than usual. The Michigan presidential primary saw a 97% increase in absentee ballot requests over their previous level from 2016. It is clear that jurisdictions will see record rates of mail and absentee voting in 2020. [...] A dramatic increase in mail or absentee ballots can lead to numerous challenges related to processing ballots, and further amplifies the need for a positive working relationship with the postal service and vendors. Increased use of mail and absentee ballots in the primary and lessons learned from states that conduct all elections by mail have helped to identify some of these potential issues before November."
U.S. Election Assistance Commission: https://www.eac.gov/