They simply can't be secured, and in every industry in the world, that would render it uncertifiable. Our election processes have early voting and an absentee ballot process for those who have potential issues with exercising their right on Election Day. This mail-in process has become such a cottage industry and is having far too profound an effect on election results.
I'll say it again, the ONLY reason to support it so enthusiastically is to have the opportunity to commit fraud. Every other justification in its favor can be served by an alternative that is already available in the process.