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