mailcl.docusign.net has no CNAME records.
Question and response
Question
dig mailcl.docusign.net. NS
Answer
The response has no answer records.
Authority
docusign.net. 101 SOA a1-246.akam.net. hostmaster.akamai.com. 2014053594 3600 1800 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0