mail.stableserver.net has no CNAME records.
Question and response
Question
dig mail.stableserver.net. NS
Answer
The response has no answer records.
Authority
stableserver.net. 3600 SOA ns1.stableserver.net. postmaster.stableserver.net. 2026022401 3600 3600 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0