A new report has revealed how vulnerable voting by mail can be. It should alarm all who are pushing for all-mail elections: Based on federal data from the U.S. Election Assistance Commission, millions of mail ballots were never counted as completed votes.

According to the commission’s 2016 report, for example, more mail ballots were misdirected and unaccounted for than the margin of votes between Hillary Clinton and Donald J. Trump. She had 2.9 million more votes, yet 6.5 million ballots were misdirected or unaccounted for by the states. In other words, for every vote that Hillary won over the eventual president nationally, more than twice as many mail ballots disappeared or went to the wrong addresses.