mx.softlayer.com has no NS records.
NS records for this zone are defined at softlayer.com.
NS records for this zone are defined at softlayer.com.
Question and response
Question
dig mx.softlayer.com. NS
Answer
The response has no answer records.
Authority
softlayer.com. 900 SOA ns1.softlayer.com. root.softlayer.com. 2025102200 7200 600 1728000 900
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0