mail.icewarp.com has no CNAME records.
Question and response
Question
dig mail.icewarp.com. NS
Answer
The response has no answer records.
Authority
icewarp.com. 2560 SOA ns1.websupport.cz. admin.websupport.cz. 1772046050 7200 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0