mail.swisscom.com has no CNAME records.
Question and response
Question
dig mail.swisscom.com. NS
Answer
The response has no answer records.
Authority
swisscom.com. 600 SOA dns3.swisscom.com. admin\.dns.swisscom.com. 53616 21600 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0