smtp4.tripnet.se has no CNAME records.
Question and response
Question
dig smtp4.tripnet.se. NS
Answer
The response has no answer records.
Authority
tripnet.se. 900 SOA ns0.tripnet.se. hostmaster.tripnet.se. 44405 7200 720 1209600 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0