nassmtp.skillsoft.com has no CNAME records.
Question and response
Question
dig nassmtp.skillsoft.com. NS
Answer
The response has no answer records.
Authority
skillsoft.com. 9240 SOA udns1.cscdns.net. hostmaster.cscdns.net. 7456 86400 86400 86400 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0