mx02.dg-sys.net has no CNAME records.
Question and response
Question
dig mx02.dg-sys.net. NS
Answer
The response has no answer records.
Authority
dg-sys.net. 3347 SOA ns1.inexio.net. hostmaster.inexio.net. 2026030401 28800 3600 1728000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0