mail2.out.flockmail.com has no NS records.
NS records for this zone are defined at flockmail.com.
NS records for this zone are defined at flockmail.com.
Question and response
Question
dig mail2.out.flockmail.com. NS
Answer
The response has no answer records.
Authority
flockmail.com. 900 SOA ns-1609.awsdns-09.co.uk. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0