mailhost.lightning.net has no CNAME records.
Question and response
Question
dig mailhost.lightning.net. NS
Answer
The response has no answer records.
Authority
lightning.net. 6891 SOA thunder.lightning.net. internic.lightning.net. 2021081600 3600 1800 604800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0