sh23137.ispgateway.de has no MX records.
Question and response
Question
dig sh23137.ispgateway.de. MX
Answer
The response has no answer records.
Authority
ispgateway.de. 2560 SOA ns.namespace4you.de. hostmaster.ns.namespace4you.de. 1772625901 86400 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0