mail.rackanddata.com has no CNAME records.
Question and response
Question
dig mail.rackanddata.com. NS
Answer
The response has no answer records.
Authority
rackanddata.com. 2392 SOA authns1.rackanddata.com. domainadmin.rackanddata.com. 1375326455 10800 3600 604800 38400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0