patron002 -- actually I do know my mailman. I live in a rural area and pick up my mail at a PO Box. I always wish Ernie a good day and he wishes me one back. And it is exceedingly rare for ANYTHING to show up in my PO Box that doesn't have my name or my wife's name on it (or, of course, the ubiquitous "occupant" mail). That's partly because Ernie knows my first and last names, and required me to submit photo IDs for myself and my wife when we rented the box.
In other places up here, I've have rural route delivery and there, too, I knew my mailman and I very rarely got mail that wasn't for me. Sometimes I'd get a letter for one of the other mailboxes right next to mine, or sometimes for the previous occupant -- in both cases, the mailman's fingers or focus slipped.
I had a neighbor who once received a letter addressed to "Fast Eddie, Kip House" plus the city and state. Fast Eddie was his nickname; Kip was the name of the guy who built his house back in 1710 or so. Mailmen really DO know their clientele. You ought to try to get to know yours.
When I lived in New York City (about 25 years ago) in an apartment house with around 100 units, my mailbox had my name and my wife's name written on a piece of tape inside the box. And yes, I knew my mailman there, too. And almost never got mail that wasn't mine.
You're assuming that all mail is handled the same. Mail which is marked with something to the effect of "don't deliver to incorrect address" is examined with extra care. This applies to very few documents -- driver's licenses, auto registrations, etc. -- and so the Postal Service handles it differently. If voter registrations AREN'T handled in this manner, it's the fault of the state in question.
Furthermore, since the introduction of the nine-digit zip code around 1986, any organization which sends out a large amount of mail has a data base by now that can identify an address down to the apartment number and can also identify who lives at that nine-digit ZIP for the most part. It is child's play for such a system to figure out that Terrell Owens doesn't match whoever lives at 56 Main St., Apt. 5Q, and to trigger an inquiry (on the off chance that Owens actually DOES live there). And it is also child's play for such a system to identify living units with more than, say, two last names receiving mail there, and to investigate when somebody racked up five or six registrations at one address. States DO have systems like this in place already, because that's how they identify duplicate registrations, which happen quite often for innocent reasons.
Actually, just printing on an envelope the words "address correction requested" will cause the post office to flag mail sent to the wrong address and return it to the sender with the correct address (if obtainable) noted on the envelope. This is done by certain advertisers who are eager to keep track of patrons and also by political campaigns (in the early stages of the campaign) to perfect their mailing lists. There is a charge for this, since it involves extra work for the USPS. I once worked on a campaign where the candidate used envelopes with those words on it -- they were envelopes which he used for a private foundation he ran, where he had an obvious interest in keeping track of donors. You should have seen his face when the Post Office socked him with a bill for several hundred dollars worth of corrections.
But the larger point is that voter registration fraud, while indeed a pain in the ass because state registrars have to spot it and take time to weed it out, has NEVER translated into actual vote fraud AT THE POLLS to any significant degree.
David Iglesias, the New Mexico Federal DA who was fired because he wouldn't push vote fraud indictments during the 2006 election, points out that there simply weren't any winnable cases to present, as agreed by him, by the local FBI office (which did the investigation), and by the Justice Dept. officials following the situation in Washington. I lived in New Mexico for five years -- it used to have some of the most outrageous vote fraud in the country but that was NEVER been accomplished through registration fraud. It's done by controlling the polling places in remote rural areas and reporting fictional numbers. There's a reason for this, of course -- it's a lot cheaper than to have to pay people to collect and use false registrations.