ftmail2.flexential.com has no NS records.
NS records for this zone are defined at flexential.com.
NS records for this zone are defined at flexential.com.
Question and response
Question
dig ftmail2.flexential.com. NS
Answer
The response has no answer records.
Authority
flexential.com. 3600 SOA ns1.peak10.com. dev-notices.peak10.com. 2018011800 3600 1200 1209600 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0