smtp.openit.de has no CNAME records.
Question and response
Question
dig smtp.openit.de. NS
Answer
The response has no answer records.
Authority
openit.de. 900 SOA ns1.plusserver.com. hostmaster.plusserver.com. 2026020403 1800 900 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0