mail.fastmail.com has no CNAME records.
Question and response
Question
dig mail.fastmail.com. NS
Answer
The response has no answer records.
Authority
fastmail.com. 2904 SOA fmns1.messagingengine.com. postmaster.messagingengine.com. 2026021600 86118 600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0