mail.zummer.su has no CNAME records.
Question and response
Question
dig mail.zummer.su. NS
Answer
The response has no answer records.
Authority
zummer.su. 3434 SOA ns1.svols.com. admin.svols.com. 2025121700 3600 900 3600000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0