ruban.smtp.rzone.de has no CNAME records.
Question and response
Question
dig ruban.smtp.rzone.de. NS
Answer
The response has no answer records.
Authority
rzone.de. 1300 SOA karlsruhe.nic.xlink.net. de-domain.xlink.net. 2026030600 3600 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0