postoffice.hostrocket.com has no CNAME records.
Question and response
Question
dig postoffice.hostrocket.com. NS
Answer
The response has no answer records.
Authority
hostrocket.com. 1769 SOA elinore.ns.cloudflare.com. dns.cloudflare.com. 2392798026 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0