mailsec.protonmail.ch has no CNAME records.
Question and response
Question
dig mailsec.protonmail.ch. NS
Answer
The response has no answer records.
Authority
protonmail.ch. 1200 SOA ns1.protonmail.ch. support.protonmail.ch. 2511171744 1200 144 1814400 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0