tb.iss.as9143.net has no MX records.
Question and response
Question
dig tb.iss.as9143.net. MX
Answer
The response has no answer records.
Authority
tb.iss.as9143.net. 900 SOA ns1.as9143.net. dnsadmin.as9143.net. 142 10800 3600 2419200 900
Additional
. 0 OPT ; payload 1400, xrcode 0, version 0, flags 0