mail.zth.netdesignhost.com has no CNAME records.
Question and response
Question
dig mail.zth.netdesignhost.com. NS
Answer
The response has no answer records.
Authority
netdesignhost.com. 999 SOA win-alfftollgm4. webmaster.netdesignhost.com. 20093135 900 600 86400 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0