mail.premiumwebservers.com has no CNAME records.
Question and response
Question
dig mail.premiumwebservers.com. NS
Answer
The response has no answer records.
Authority
premiumwebservers.com. 3600 SOA nxg9180.premiumwebservers.com. hostmaster.premiumwebservers.com. 132 900 600 86400 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0