mail1.itxplain.de has no CNAME records.
Question and response
Question
dig mail1.itxplain.de. NS
Answer
The response has no answer records.
Authority
itxplain.de. 8668 SOA ns1.mainns.de. info.domains.resellerinterface.de. 1772668800 14400 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0