mail.protonmail.ch has no NS records.
NS records for this zone are defined at protonmail.ch.
NS records for this zone are defined at protonmail.ch.
Question and response
Question
dig mail.protonmail.ch. NS
Answer
The response has no answer records.
Authority
protonmail.ch. 1200 SOA ns1.protonmail.ch. support.protonmail.ch. 2511171741 1200 144 1814400 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0