mail6.out.flockmail.com has no CNAME records.
Question and response
Question
dig mail6.out.flockmail.com. NS
Answer
The response has no answer records.
Authority
flockmail.com. 735 SOA ns-1609.awsdns-09.co.uk. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0