ns28.1and1.com.onlinehome-server.com has no MX records.
Question and response
Question
dig ns28.1and1.com.onlinehome-server.com. MX
Answer
The response has no answer records.
Authority
onlinehome-server.com. 30 SOA ns-1and1.ui-dns.com. dnsadmin.ionos.com. 2014969356 14400 3600 605000 30
Additional
. 0 OPT ; payload 1220, xrcode 0, version 0, flags 0