smtp-relay.dcsix.net has no MX records.
Question and response
Question
dig smtp-relay.dcsix.net. MX
Answer
The response has no answer records.
Authority
dcsix.net. 300 SOA ns1.dcsix.net. hostmaster.dcsix.net. 2015120640 14400 3600 604800 300
Additional
. 0 OPT ; payload 1400, xrcode 0, version 0, flags 0