b.gmoregistry.net has no MX records.
Question and response
Question
dig b.gmoregistry.net. MX
Answer
The response has no answer records.
Authority
gmoregistry.net. 3600 SOA ns1.gmoregistry.net. noc.gmoregistry.net. 80133 3600 900 604800 3600
Additional
. 0 OPT ; payload 1280, xrcode 0, version 0, flags 0