lk.sdstest.su has no MX records.
Question and response
Question
dig lk.sdstest.su. MX
Answer
The response has no answer records.
Authority
sdstest.su. 10800 SOA ns1.reg.ru. hostmaster.ns1.reg.ru. 1768943610 14400 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0