s.smailru.net has no MX records.
Question and response
Question
dig s.smailru.net. MX
Answer
The response has no answer records.
Authority
s.smailru.net. 300 SOA ns1.mail.ru. hostmaster.mail.ru. 3563223034 300 900 1209600 300
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0