firewall.sielkamp.etc.tu-bs.de has no MX records.
Question and response
Question
dig firewall.sielkamp.etc.tu-bs.de. MX
Answer
The response has no answer records.
Authority
sielkamp.etc.tu-bs.de. 300 SOA rzdnsprime.rz.tu-bs.de. noc.tu-bs.de. 706290628 21600 3600 1814400 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0