a.subnet.rcn.com has no MX records.
Question and response
Question
dig a.subnet.rcn.com. MX
Answer
The response has no answer records.
Authority
a.subnet.rcn.com. 86400 SOA master.dns.rcn.net. hostmaster.rcn.net. 2026012701 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0