mx00.ionos.com has no CNAME records.
Question and response
Question
dig mx00.ionos.com. NS
Answer
The response has no answer records.
Authority
ionos.com. 537 SOA ns-1and1.ui-dns.com. dnsadmin.1and1.com. 2018064833 14400 3600 605000 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0