sp6.zaptail.com has no CNAME records.
Question and response
Question
dig sp6.zaptail.com. NS
Answer
The response has no answer records.
Authority
zaptail.com. 154 SOA ns1.dnsexit.com. admin.netdorm.com. 2000060701 86000 180000 1814400 1200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0