mail.sensenet.com has no CNAME records.
Question and response
Question
dig mail.sensenet.com. NS
Answer
The response has no answer records.
Authority
sensenet.com. 377 SOA ns1-01.azure-dns.com. support.sensenet.com. 2012042273 28800 14400 1209600 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0