correo.plexus.es has no CNAME records.
Question and response
Question
dig correo.plexus.es. NS
Answer
The response has no answer records.
Authority
plexus.es. 1800 SOA bob.ns.cloudflare.com. dns.cloudflare.com. 2397421609 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0