mailserver.securewebportal.net has no CNAME records.
Question and response
Question
dig mailserver.securewebportal.net. NS
Answer
The response has no answer records.
Authority
securewebportal.net. 830 SOA ns1.efscloud.net. efscloud.net.domainsbyproxy.com. 41 600 300 3600000 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0