oxygen.ns.hetzner.com has no MX records.
Question and response
Question
dig oxygen.ns.hetzner.com. MX
Answer
The response has no answer records.
Authority
hetzner.com. 7200 SOA ns1.your-server.de. postmaster.your-server.de. 2026021900 14400 10800 604800 7200
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0