smtp.interdomain.org has no CNAME records.
Question and response
Question
dig smtp.interdomain.org. NS
Answer
The response has no answer records.
Authority
interdomain.org. 300 SOA ns1.interdomain.net. admin.interdomain.net. 467112996 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0