I don't know HOW FEASIBLE this would be, but I haven't seen this mentioned so I thought I would go ahead and suggest this:
If you are strictly in the US... get a huge database of all zip codes, states, cities and streets. Now look for these in your addresses. You can validate what you find by testing if, say, the city you found exists in the state you found, or by checking if the street you found exists in the city you found. If not, chances are John isn't for John's street, but is the name of the addressee... Basically, get the most information you can and check your addresses against it.
An extreme example would be to get A LIST OF ALL THE ADDRESSES IN THE US OF A and then find which one has the most relevant match to each of your addresses...
