ns4.rockstable.org has no MX records.
Question and response
Question
dig ns4.rockstable.org. MX
Answer
The response has no answer records.
Authority
rockstable.org. 43200 SOA ns3.rockstable.org. hostmaster.rockstable.it. 2022015234 86400 3600 2419200 43200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0