mail.server.hostnexus.com has no CNAME records.
Question and response
Question
dig mail.server.hostnexus.com. NS
Answer
The response has no answer records.
Authority
server.hostnexus.com. 10502 SOA ns.server.hostnexus.com. lflynn.hostnexus.com. 1619416151 10800 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0