mail.dc.unit-is.com has no CNAME records.
Question and response
Question
dig mail.dc.unit-is.com. NS
Answer
The response has no answer records.
Authority
dc.unit-is.com. 3260 SOA h102.unit-is.com. admin.unit-is.com. 2017013106 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0