The following should work, provided OurThing points to the generated strongly-typed resource class, for looking up localized strings, etc, and "EmailUniqueError" is one of the static properties of this class:

    public class UniquenessAttribute : ValidationAttribute
    {
        public UniquenessAttribute()
        {
            ErrorMessageResourceName = "EmailUniqueError";
            ErrorMessageResourceType = typeof (Resources.OurThing);
        }

        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            if (failure) // to implement
                return new ValidationResult(FormatErrorMessage(validationContext.DisplayName));
            ...
        }
    }

Also make sure you're validating all properties (if you use TryValidateObject for example, make sure the last parameter is set to true) when doing your tests.