nassmtp.skillsoft.com has no MX records.
Question and response
Question
dig nassmtp.skillsoft.com. MX
Answer
The response has no answer records.
Authority
skillsoft.com. 86400 SOA udns1.cscdns.net. hostmaster.cscdns.net. 7454 86400 86400 86400 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0