mail-sp-imap.serverplan.com has no CNAME records.
Question and response
Question
dig mail-sp-imap.serverplan.com. NS
Answer
The response has no answer records.
Authority
serverplan.com. 3600 SOA ns1.dns4userver.com. server.serverplan.com. 2026022500 10800 3600 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0