_netblocks.sparkpostmail.com has no CNAME records.
Question and response
Question
dig _netblocks.sparkpostmail.com. NS
Answer
The response has no answer records.
Authority
sparkpostmail.com. 878 SOA ns-1855.awsdns-39.co.uk. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0