a2631.mx.srv.dfn.de has no CNAME records.
Question and response
Question
dig a2631.mx.srv.dfn.de. NS
Answer
The response has no answer records.
Authority
dfn.de. 3600 SOA ns1.dfn.de. hostmaster.dfn.de. 1772721194 28800 7200 3024000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0