mx.rcn.com has no CNAME records.
Question and response
Question
dig mx.rcn.com. NS
Answer
The response has no answer records.
Authority
rcn.com. 70267 SOA master.dns.rcn.net. hostmaster.rcn.com. 2026022201 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0