shell.pubnix.net has no MX records.
Question and response
Question
dig shell.pubnix.net. MX
Answer
The response has no answer records.
Authority
pubnix.net. 86400 SOA ns1.pubnix.net. root.pubnix.net. 2025084924 3600 900 2419200 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0