mx.dotster.com has no CNAME records.
Question and response
Question
dig mx.dotster.com. NS
Answer
The response has no answer records.
Authority
dotster.com. 24 SOA ns1.dotster.com. ops.dotster.com. 1333639566 16384 2048 1048576 2560
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0