Check out my report for the ACLU of Florida, “Vote-By-Mail Ballots Cast in Florida.” Vote-by-mail ballots cast in the 2012 and 2016 general election had a higher rejection rate than votes cast at assigned precincts on Election Day and at early voting sites, and more importantly, younger voters and racial and ethnic minority voters were much more likely to cast mail ballots that were rejected and were less likely to have their ballots cured.
Full report is available here.
ElectionSmith 