ns6.dhl.com has no CNAME records.
Question and response
Question
dig ns6.dhl.com. NS
Answer
The response has no answer records.
Authority
dhl.com. 300 SOA ns4.dhl.com. netmaster.dhl.com. 2016118539 10800 3600 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0