mailman.dca.net has no CNAME records.
Question and response
Question
dig mailman.dca.net. NS
Answer
The response has no answer records.
Authority
dca.net. 1685 SOA dns.dca.net. root.dca.net. 2026011801 10800 3600 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0