EasyPost offers the best RESTful JSON International Address Verification API. We also offer up-to-date client libraries for .Net, Java, Node, PHP, Python, Ruby, and more.
If you're not using address verification it's likely hurting your bottom line. You're probably losing money by shipping to addresses that aren't deliverable. If you're doing this abroad, it's even more costly.
By using the EasyPost Address Verification API to provide real time address verification the customer can confirm their address is shippable before the package is shipped, not after.