mail.skylinux.com has no CNAME records.
Question and response
Question
dig mail.skylinux.com. NS
Answer
The response has no answer records.
Authority
skylinux.com. 3524 SOA ns.skylinux.org. domain.skylinux.org. 2014012803 43200 10800 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0