mx.o2online.de has no CNAME records.
Question and response
Question
dig mx.o2online.de. NS
Answer
The response has no answer records.
Authority
o2online.de. 581 SOA ns1.o2online.de. domain-de.o2.com. 2026022401 10800 3600 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0