mail.iristel.com has no NS records.
NS records for this zone are defined at iristel.com.
NS records for this zone are defined at iristel.com.
Question and response
Question
dig mail.iristel.com. NS
Answer
The response has no answer records.
Authority
iristel.com. 86400 SOA pdns104.ultradns.com. tldadmin.iristel.com. 2020063469 86400 86400 86400 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0