smtp.dsl.systemec.nl has no CNAME records.
Question and response
Question
dig smtp.dsl.systemec.nl. NS
Answer
The response has no answer records.
Authority
dsl.systemec.nl. 2452 SOA ns.systemec.nl. postmaster.dsl.systemec.nl. 2020063001 28800 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0