mx.iks-jena.de has no CNAME records.
Question and response
Question
dig mx.iks-jena.de. NS
Answer
The response has no answer records.
Authority
iks-jena.de. 12387 SOA avalon.iks-jena.de. hostmaster.iks-jena.de. 2026030403 28800 3600 3600000 57600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0