designate/designate/resources/schemas/v2/rdata/mx.json

{
    "$schema": "http://json-schema.org/draft-04/hyper-schema",

    "additionalProperties": false,

    "required": ["preference", "exchange"],

    "properties": {
        "preference": {
            "type": "integer"
        },
        "exchange": {
            "type": "string",
            "format": "hostname"
        }
    }
}