According to the law and court ruling, the clerk may accept ballots that are postmarked up to four days after Election Day and ballots without a postmark up to three days after Election Day.
According to the court:
"“If a voter properly and timely casts their vote by mailing their ballot before or on the day of the election, and through a post office omission the ballot is not postmarked, it would go against public policy to discount that properly cast vote,” the justices wrote in their order.
“Rejecting timely mail ballots because of postal service omissions cuts against the strong public interest in exercising the right to vote,” the justices said. “Notably, the RNC presented no evidence or allegations that counting mail ballots without postmarks under [state law] would be subject to voter fraud, or that the election security measures currently in place are inadequate to address its concerns regarding these ballots.”
(source)
The logic here is that if a ballot arrives within three days after Election Day, it was almost certainly mailed before Election Day given the speed of the mail. If the Post Office neglected to postmark it, that is not the fault of the voter.
Additionally, once a ballot is received by the clerk, it still undergoes verification before being counted.