mx.stackmail.com has no CNAME records.
Question and response
Question
dig mx.stackmail.com. NS
Answer
The response has no answer records.
Authority
stackmail.com. 188 SOA ns1.stackdns.com. hostmaster.stackdns.com. 1666943089 1800 900 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0