mail.iristel.com has no CNAME records.
Question and response
Question
dig mail.iristel.com. NS
Answer
The response has no answer records.
Authority
iristel.com. 10800 SOA pdns104.ultradns.com. tldadmin.iristel.com. 2020063469 86400 86400 86400 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0