mail.first-ns.de has no MX records.
Question and response
Question
dig mail.first-ns.de. MX
Answer
The response has no answer records.
Authority
first-ns.de. 601 SOA ns1.your-server.de. postmaster.your-server.de. 2025120200 86400 1800 3600000 601
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0