This is it.

“When ballots are submitted to USPS in large-volume batches, if any one ballot in the batch cannot be verified against the Portal, all ballots in that batch will be rejected. For example, if a state election official brings a batch of 10,000 ballots to USPS and USPS is unable to match just one of those ballots against the Portal—because, for example, someone has recently changed their name after marriage or they’ve moved—then USPS would refuse to mail the remaining 9,999 ballots as well.”