nsb.retn.net has no MX records.
Question and response
Question
dig nsb.retn.net. MX
Answer
The response has no answer records.
Authority
retn.net. 86400 SOA nsa.retn.net. noc.retn.net. 2026020400 7200 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0