mail4.link11.com has no CNAME records.
Question and response
Question
dig mail4.link11.com. NS
Answer
The response has no answer records.
Authority
link11.com. 38 SOA ns1.link11.de. hostmaster.link11.de. 1771858282 16384 2048 1048576 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0