mail.sierracases.com has no CNAME records.
Question and response
Question
dig mail.sierracases.com. NS
Answer
The response has no answer records.
Authority
sierracases.com. 1560 SOA suzanne.ns.cloudflare.com. dns.cloudflare.com. 2398241186 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0