mail.caritas-mettmann.de has no CNAME records.
Question and response
Question
dig mail.caritas-mettmann.de. NS
Answer
The response has no answer records.
Authority
caritas-mettmann.de. 278 SOA ns1077.ui-dns.de. hostmaster.schlund.de. 2017062935 28800 7200 604800 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0