Don't they compare the number of ballots sent compared to the number received?
Yes. The number sent is public records and the number received for counting, (under the state laws which vary from state to state re: postmarked prior to close of election, received on or before election day, postmarked on or before election day and received 2,3,5,10,14 days after election day).
All that is fine and dandy, but, assume 1,000,000 ballots sent and only 750,000 received, what happened to the other 250,000? Lost in the mail, never voted and returned by the voter, destroyed in fire, flood, etc.?
That is where the post office personnel come in, but how do you prove nefarious actions? Without a confession, you can’t?