mail20.swisscom.com has no NS records.
NS records for this zone are defined at swisscom.com.
NS records for this zone are defined at swisscom.com.
Question and response
Question
dig mail20.swisscom.com. NS
Answer
The response has no answer records.
Authority
swisscom.com. 600 SOA dns3.swisscom.com. admin\.dns.swisscom.com. 53631 21600 3600 604800 600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0