mail.certifiedmail.com has no CNAME records.
Question and response
Question
dig mail.certifiedmail.com. NS
Answer
The response has no answer records.
Authority
certifiedmail.com. 3600 SOA ns1-01.azure-dns.com. azuredns-hostmaster.microsoft.com.certifiedmail.com. 30 3600 60 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0