o2.sgmail.github.com has no CNAME records.
Question and response
Question
dig o2.sgmail.github.com. NS
Answer
The response has no answer records.
Authority
github.com. 890 SOA ns-1707.awsdns-21.co.uk. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0