mail.rdtc.ru has no MX records.
Question and response
Question
dig mail.rdtc.ru. MX
Answer
The response has no answer records.
Authority
rdtc.ru. 3600 SOA ns.rdtc.ru. root.rdtc.ru. 3008041042 5400 1800 604800 7200
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0