mx00.ionos.com has no MX records.
Question and response
Question
dig mx00.ionos.com. MX
Answer
The response has no answer records.
Authority
ionos.com. 600 SOA ns-1and1.ui-dns.com. dnsadmin.1and1.com. 2018064838 14400 3600 605000 600
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0