mail.serverplan.com has no CNAME records.
Question and response
Question
dig mail.serverplan.com. NS
Answer
The response has no answer records.
Authority
serverplan.com. 2760 SOA ns1.dns4userver.com. server.serverplan.com. 2025120200 10800 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0