mta-sw146.gcb.su has no MX records.
Question and response
Question
dig mta-sw146.gcb.su. MX
Answer
The response has no answer records.
Authority
gcb.su. 3600 SOA ns-mail02.getcourse.ru. hostmaster.gcb.su. 668543 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1680, xrcode 0, version 0, flags 0