o6.sgmail.github.com has no CNAME records.
Question and response
Question
dig o6.sgmail.github.com. NS
Answer
The response has no answer records.
Authority
github.com. 3514 SOA dns1.p08.nsone.net. hostmaster.nsone.net. 1656468023 43200 7200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0