ns.EX-SIS.RU has no MX records.
Question and response
Question
dig ns.EX-SIS.RU. MX
Answer
The response has no answer records.
Authority
ex-sis.ru. 1440 SOA ns.ex-sis.ru. admin.pledpodushkin.ru. 44 10800 3600 604800 604800
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0