Absentee/mail-in ballots must meet a variety of state requirements to be verified and counted. In the 2020 election, all 50 states require voters to provide valid signatures on their absentee/mail-in ballot return documents. Thirty-two of these require election workers to match a voter’s signature on ballot return documents with the signature on record for that voter.
In the event that a ballot missing a signature or there is a discrepancy in signature matching, eighteen states require officials to notify voters and allow voters to correct signature errors through a process called ballot curing.  Ballot curing is a two-part process that involving notification and correction. States that do not have a ballot curing process do not count ballots with missing or mismatched signatures.
This article lists state laws on ballot curing in the 18 states with ballot curing provisions.