Voting by mail is increasingly popular. Although voters enjoy the convenience of voting by mail, the public is unaware how prone to error mail ballots are. Election administrations reject mail ballots for a variety of reasons. Because German electoral laws treat rejected ballots as not having been cast, the reported number of invalid mail ballots underestimates the true share of uncounted mail ballots. Between 3.2% and 4.0% of the mailed ballots in German elections are estimated to remain uncounted, which is far higher than the share of invalid in-person votes. This paper argues that rejected mail ballots should be reported in the official records to make voters aware of the risks of an uncounted mail ballot.