_netblocks.swishmail.com has no CNAME records.
Question and response
Question
dig _netblocks.swishmail.com. NS
Answer
The response has no answer records.
Authority
swishmail.com. 3493 SOA ns1.webgoku.com. hostmaster.swishmail.com. 2026022201 86400 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0