unknown.servercentral.net has no MX records.
Question and response
Question
dig unknown.servercentral.net. MX
Answer
The response has no answer records.
Authority
servercentral.net. 2560 SOA ns1.servercentral.net. dns.servercentral.net. 2025110302 16384 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0