The difference between absentee voting and mail-only voting is that absentee voting requires a request for a ballot, but mail-only voting means a ballot will be sent to a house where a voter once lived. This article provides a great primer on why this is an invitation for fraud, and also uses Honest Elections Project data to show how widespread voter registration inaccuracies truly are.

Source: Inaccurate voter registration is an incubator for fraud