mail.dynu.com has no MX records.
Question and response
Question
dig mail.dynu.com. MX
Answer
The response has no answer records.
Authority
dynu.com. 3600 SOA ns1.dynu.com. administrator.dynu.com. 76193390 1800 300 86400 1800
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0