There are ways to stop voter fraud that uses undelivered ballots
Jay Valentine makes a strong case that ballots should not be mailed out to addresses for which mail cannot be delivered (vacant lots), at which voters cannot reside (commercial or industrial addresses), or where mail cannot be accurately delivered (apartment or dorm buildings with no apartment number or dorm room number). He recommends purifying voter rolls so that ballots will not be mailed out to addresses where mail cannot be delivered or voters cannot live. This can be done by compiling the Undeliverable Ballot Database (UBD).
To support his argument, Valentine quotes a message from a retired mailman who said that he saw hundreds, or even thousands, of returned ballots put in a basket at the post office for "returned/undeliverable mail" until someone came by and picked them up. It is important to note that these mailed-out ballots are "live ammunition" that can be voted by anyone who picks them up.
Some states (such as Texas) require that the voter include parts of a social security number or driver's license number in the returned ballot. However, this is certainly not a foolproof method, as the requested data are accessible via commercial databases, and fraudsters can find and supply it. This is also a good reason to keep voter signatures away from public view because that, too, makes it too easy for fraudsters. It is a constant battle of defense versus offense in the voter fraud arena.
Other states have no limitations. Michigan has opened the floodgates to fraudulent voting. The GOP will be at a severe disadvantage in that state due to the changes in the voting laws that have been enacted.
An additional way to attack the problem is to put some constraints on the post office's handling of returned ballots. It would be great to get all the bogus addresses off the voter rolls, but what shall we do with the ones that slip through?
One approach is to bring a lawsuit requiring the post office to keep a record of the number of returned ballots, place them in a safe location, and have the stored returned ballots made available for inspection by representatives of both parties to ensure that no ballots are removed. A federal injunction could see that this practice is followed nationwide. If talk about the "sanctity of elections" is to be backed up with actions, such a move should not be opposed.
Another possible approach would be an injunction banning mailing out ballots unless it's verified that an actual voter lives at an address. In other words, states would be enjoined from mailing ballots to addresses on the Undeliverable Ballot Database. Of course, judges would have to be convinced that the information on the UBD actually belongs there. But as Valentine shows, that shouldn't be difficult.
Another policy (or injunction) that would help curtail the fraud in voting by mail would be to have the government mail the ballots via certified mail, with a return receipt required from the voter. Alternatively, if voters pick up a ballot for later mail-in voting, they will have to show ID to do so. This would be another check on the mass mailing of ballots with all the attendant security problems.
Of course, none of these safeguards should be a substitute for the position that federal law requires that elections must be held "[t]he Tuesday next after the 1st Monday in November, in every numbered year" (2 U.S. Code § 7), both for presidents and members of Congress. In addition to being legally correct, something Deroy Murdock establishes, multi-week or even multi-month elections make it much easier to cheat. The extra time helps establish who have not voted or will not vote and substitute bogus ballots on their behalf.