Of the half dozen or so I tested (not super scientifically, but with a moderately sized sample dataset that I ran through several APIs), Smarty seemed to be the

Of the half dozen or so I tested (not super scientifically, but with a moderately sized sample dataset that I ran through several APIs), Smarty seemed to be the most accurate: https://www.smarty.com/ https://www.smarty.com/ It was expensive though. And there's the Google Maps API, of course:https://developers.google.com/maps/documentation/address-validation/overview https://developers.google.com/maps/documentation/address-val...