Mail-ins should not be allowed anyway; they aren't legal in just about every other country. The absentee system, in which people have to request a ballot far in advance, supply a good reason such as a diplomatic, military, or other job overseas, would yield a small enough amount of ballots that another system could be used to verify them without exposing the individuals' identities. It wouldn't be like mail-in ballots, where they can just swamp the system in unverifiable, fraudulent bullshit.