mail.dynu.com has no CNAME records.
Question and response
Question
dig mail.dynu.com. NS
Answer
The response has no answer records.
Authority
dynu.com. 3600 SOA ns1.dynu.com. administrator.dynu.com. 76198245 1800 300 86400 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0