smtp4.tng.de has no MX records.
Question and response
Question
dig smtp4.tng.de. MX
Answer
The response has no answer records.
Authority
tng.de. 1800 SOA ns1.ennit.net. hostmaster.tng.de. 2026022601 14400 1800 3600000 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0