customer-net.de has no MX records.
Question and response
Question
dig customer-net.de. MX
Answer
The response has no answer records.
Authority
customer-net.de. 3600 SOA rdns1.internetx.de. ripe.internetx.de. 2025110600 10000 3600 2419200 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0