I currently serve as an election officer for my county board of elections.  Part of my duty is to ensure proper chain of custody of the ballot from the time the voter arrives to request a ballot through the time that ballot is scanned.  I am also charged with ensuring no voter casts a ballot once the polls have closed.  The absentee ballot process needs to ensure a similar chain of custody.  Accepting an absentee ballot without a post mark or proof of delivery on time compromises both the chain of custody (from the voter to the ballot box) and the timely casting of a vote.  The latter is inherently unfair to voters who show up in person to cast a ballot on election day, given the possibility that those ballots missing a post mark could have been completed after the polls have closed on Tuesday at 7 PM.