mx1.fastmail.gr has no CNAME records.
Question and response
Question
dig mx1.fastmail.gr. NS
Answer
The response has no answer records.
Authority
fastmail.gr. 1800 SOA alec.ns.cloudflare.com. dns.cloudflare.com. 2394115862 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0