cable.rcncustomer.com has no MX records.
Question and response
Question
dig cable.rcncustomer.com. MX
Answer
The response has no answer records.
Authority
cable.rcncustomer.com. 86400 SOA master.dns.rcn.net. hostmaster.rcn.com. 2017071802 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0