o5.sgmail.github.com has no CNAME records.
Question and response
Question
dig o5.sgmail.github.com. NS
Answer
The response has no answer records.
Authority
github.com. 3600 SOA dns1.p08.nsone.net. hostmaster.nsone.net. 1656468023 43200 7200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0