What response is given for "undeliverable" in the API?

Quick answer, if the address is invalid, we return an empty set: [ ].

Here is the JSON result for a deliverable address:

[
    {
        "input_index": 0,
        "candidate_index": 0,
        "delivery_line_1": "1600 Amphitheatre Pkwy",
        "last_line": "Mountain View CA 94043-1351",
        "delivery_point_barcode": "940431351000",
        "components": {
            "primary_number": "1600",
            "street_name": "Amphitheatre",
            "street_suffix": "Pkwy",
            "city_name": "Mountain View",
            "state_abbreviation": "CA",
            "zipcode": "94043",
            "plus4_code": "1351",
            "delivery_point": "00",
            "delivery_point_check_digit": "0"
        },
        "metadata": {
            "record_type": "S",
            "county_fips": "06085",
            "county_name": "Santa Clara",
            "carrier_route": "C058",
            "congressional_district": "14"
        },
        "analysis": {
            "dpv_match_code": "Y",
            "dpv_footnotes": "AABB",
            "dpv_cmra": "N",
            "dpv_vacant": "N",
            "ews_match": false,
            "footnotes": "N#"
        }
    }
]

Here is the JSON result for a VACANT address:

[
    {
        "input_index": 0,
        "candidate_index": 0,
        "delivery_line_1": "2559 Plum St",
        "last_line": "Brook Park MN 55007-4641",
        "delivery_point_barcode": "550074641594",
        "components": {
            "primary_number": "2559",
            "street_name": "Plum",
            "street_suffix": "St",
            "city_name": "Brook Park",
            "state_abbreviation": "MN",
            "zipcode": "55007",
            "plus4_code": "4641",
            "delivery_point": "59",
            "delivery_point_check_digit": "4"
        },
        "metadata": {
            "record_type": "S",
            "county_fips": "27065",
            "county_name": "Kanabec",
            "carrier_route": "R001",
            "congressional_district": "08"
        },
        "analysis": {
            "dpv_match_code": "Y",
            "dpv_footnotes": "AABB",
            "dpv_cmra": "N",
            "dpv_vacant": "Y",
            "ews_match": false,
            "footnotes": "N#"
        }
    }
]

Here is the JSON result for an invalid address:

[]