mail.iai-system.com has no CNAME records.
Question and response
Question
dig mail.iai-system.com. NS
Answer
The response has no answer records.
Authority
iai-system.com. 300 SOA dns1.iai-system.com. root.iai-system.com. 1772110904 3600 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0