mail3.nrw.net has no MX records.
Question and response
Question
dig mail3.nrw.net. MX
Answer
The response has no answer records.
Authority
nrw.net. 300 SOA a.ns.nrw.net. hostmaster.joker.com. 2026010551 14400 3600 1209600 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0