ns.b-ix.net has no MX records.
Question and response
Question
dig ns.b-ix.net. MX
Answer
The response has no answer records.
Authority
b-ix.net. 600 SOA ns.b-ix.net. hostmaster.b-ix.net. 2025120701 3600 600 86400 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0