Over 300K mail ballots were rejected in the 2016 November election. We might see a million or more rejected in 2020 because of higher usage and more people who are unfamiliar with the process https://www.washingtonpost.com/politics/tens-of-thousands-of-mail-ballots-have-been-tossed-out-in-this-years-primaries-what-will-happen-in-november/2020/07/16/fa5d7e96-c527-11ea-b037-f9711f89ee46_story.html

Only 19 states authorize (not mandate) election officials to contact voters about ballot problems. Four of these states explicitly contact voters by mail only -- and more states can use mail. Not clear if people will be notified in time to fix problems https://twitter.com/AP/status/1283593211483697153

This story notes poor data management practices in Georgia...which could mean election officials fail to notify voters with problem mail ballots (even if they've reached a legal agreement to do so). Campaigns can't assist if there is bad data https://www.ajc.com/news/state--regional-govt--politics/absentee-voting-program-embraced-georgia-voters-then-abandoned/hkNttNsgXlaYZXjUatnvjK/