domains.proton.ch has no MX records.
Question and response
Question
dig domains.proton.ch. MX
Answer
The response has no answer records.
Authority
domains.proton.ch. 1200 SOA ns1-domains.proton.ch. support.protonmail.com. 2003180915 1200 144 604800 3600
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0