mx3.mail.icloud.com has no CNAME records.
Question and response
Question
dig mx3.mail.icloud.com. NS
Answer
The response has no answer records.
Authority
icloud.com. 46 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