smtp.courseware.nl has no CNAME records.
Question and response
Question
dig smtp.courseware.nl. NS
Answer
The response has no answer records.
Authority
courseware.nl. 300 SOA ns0.transip.net. hostmaster.transip.nl. 2026022400 86400 1800 2419200 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0