mail.hostroommail.com has no CNAME records.
Question and response
Question
dig mail.hostroommail.com. NS
Answer
The response has no answer records.
Authority
hostroommail.com. 3216 SOA dns.hostroommail.com. hostmaster.hostroommail.com. 1472585847 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0