mail01.networking4all.net has no MX records.
Question and response
Question
dig mail01.networking4all.net. MX
Answer
The response has no answer records.
Authority
networking4all.net. 300 SOA ns0.transip.net. hostmaster.transip.nl. 2025112801 86400 1800 2419200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0