mon-mail.formassembly.com has no CNAME records.
Question and response
Question
dig mon-mail.formassembly.com. NS
Answer
The response has no answer records.
Authority
formassembly.com. 300 SOA ns-6.awsdns-00.com. awsdns-hostmaster.amazon.com. 201801091 7200 900 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0