ns2.mail.RU has no MX records.
Question and response
Question
dig ns2.mail.RU. MX
Answer
The response has no answer records.
Authority
mail.RU. 60 SOA ns1.mail.RU. hostmaster.mail.RU. 3562704350 900 900 604800 60
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0