sip.datakom.ru has no MX records.
Question and response
Question
dig sip.datakom.ru. MX
Answer
The response has no answer records.
Authority
datakom.ru. 3600 SOA ns1.luadns.net. hostmaster.luadns.com. 1772509624 1200 120 604800 3600
Additional
. 32768 OPT ; payload 512, xrcode 0, version 0, flags 32768