mx4.vist.is has no CNAME records.
Question and response
Question
dig mx4.vist.is. NS
Answer
The response has no answer records.
Authority
vist.is. 89 SOA ns-2013.awsdns-59.co.uk. awsdns-hostmaster.amazon.com. 1 7200 900 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0