Simple structure that would contain validation errors messages
Naming
Class names in C#, whether they're for a static, non-static, internal, public, private, abstract, nested or on-its-own class, should be PascalCase.
Find the intruder:
public class _Notice
public class _Exporter
public class _Importer
public class _Shipping
public class Address


 Only Address is correct!

Keep the underscore prefix for private fields, it seems you're using prefixes only to avoid name clashes - this smells, because in naming what's important is consistency - using arbitrary prefixes is bad practice.
Also, it's possible the nested types are not warranted, but it's very hard to tell without seeing how you're using this ValidationError static class.