sendmail.dns-net.de has no MX records.
Question and response
Question
dig sendmail.dns-net.de. MX
Answer
The response has no answer records.
Authority
dns-net.de. 86400 SOA ns0.dns-net.de. hostmaster.dns-net.de. 2026021301 10800 3600 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0