ns4.smtp2go.com has no CNAME records.
Question and response
Question
dig ns4.smtp2go.com. NS
Answer
The response has no answer records.
Authority
smtp2go.com. 600 SOA ns2.smtpcorp.com. hostmaster.smtp2go.com. 2015074662 14400 3600 1209600 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0