Where can I get list of valid US zip codes for free? I found this list but I do not think it is accurate.

https://gist.github.com/dryan/7486408

For example, 34001 and 04857 are listed a valid in the above link but USPS says they are invalid zip codes. If I do not find a ready made list, I will have to use USPS api and check each zip code individually.