mx01.mail.icloud.com has no NS records.
NS records for this zone are defined at icloud.com.
NS records for this zone are defined at icloud.com.
Question and response
Question
dig mx01.mail.icloud.com. NS
Answer
The response has no answer records.
Authority
icloud.com. 300 SOA ns-ext-prod.jackfruit.apple.com. dnscontact.apple.com. 2026021101 300 300 3628800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0