mx01.ionos.es has no CNAME records.
Question and response
Question
dig mx01.ionos.es. NS
Answer
The response has no answer records.
Authority
ionos.es. 600 SOA ns-1and1.ui-dns.com. dnsadmin.1and1.com. 2018062342 14400 3600 605000 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0