I remember reading once that they start by taking an image of the envelope, finding the zip and numerical portion of the address, then searching their database

I remember reading once that they start by taking an image of the envelope, finding the zip and numerical portion of the address, then searching their database for possible matches where the numerical portion is within a particular zip. Once it has a list of possible matches the system then starts comparing the address on the image to possible matches within the list of returned matches.