rpc-extra.net has no MX records.
Question and response
Question
dig rpc-extra.net. MX
Answer
The response has no answer records.
Authority
rpc-extra.net. 86400 SOA server.rpc-extra.net. root.rpc-extra.net. 8 21600 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0