mx.5-55.ru has no CNAME records.
Question and response
Question
dig mx.5-55.ru. NS
Answer
The response has no answer records.
Authority
5-55.ru. 760 SOA speaker.5-55.ru. root.5-55.ru. 2009091075 3600 1800 420 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0