is-sys-st6.i.smailru.net has no MX records.
Question and response
Question
dig is-sys-st6.i.smailru.net. MX
Answer
The response has no answer records.
Authority
i.smailru.net. 300 SOA ns1.mail.ru. hostmaster.mail.ru. 3563684586 300 900 1209600 300
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0