mail.apple.com has no CNAME records.
Question and response
Question
dig mail.apple.com. NS
Answer
The response has no answer records.
Authority
apple.com. 223 SOA ns-ext-prod.jackfruit.apple.com. dnscontact.apple.com. 2026022006 300 300 3628800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0