github.io has no MX records.
Question and response
Question
dig github.io. MX
Answer
The response has no answer records.
Authority
github.io. 3600 SOA dns1.p05.nsone.net. hostmaster.nsone.net. 1647625169 43200 7200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0