o2.smtp.overgroup.com has no CNAME records.
Question and response
Question
dig o2.smtp.overgroup.com. NS
Answer
The response has no answer records.
Authority
overgroup.com. 60 SOA ns1-02.azure-dns.com. hostmaster.overgroup.com. 2010071006 3600 600 604800 60
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0