_spf.rcn.com has no NS records.
NS records for this zone are defined at rcn.com.
NS records for this zone are defined at rcn.com.
Question and response
Question
dig _spf.rcn.com. NS
Answer
The response has no answer records.
Authority
rcn.com. 86400 SOA master.dns.rcn.net. hostmaster.rcn.com. 2026022301 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0