cf.arhschool5.ru has no MX records.
Question and response
Question
dig cf.arhschool5.ru. MX
Answer
The response has no answer records.
Authority
arhschool5.ru. 10800 SOA ns1.reg.ru. hostmaster.ns1.reg.ru. 1759161677 14400 3600 604800 10800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0