smtp.sip.is has no CNAME records.
Question and response
Question
dig smtp.sip.is. NS
Answer
The response has no answer records.
Authority
sip.is. 1315 SOA ns01.ns.sip.is. hostmaster.sip.is. 1772808850 16384 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0