spool.mail.rcn.net has no CNAME records.
Question and response
Question
dig spool.mail.rcn.net. NS
Answer
The response has no answer records.
Authority
mail.rcn.net. 5298 SOA lists.lnh.mail.rcn.net. root.rcn.com. 2025121205 7200 1800 604800 21600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0