dns-sec.transtk.RU has no MX records.
Question and response
Question
dig dns-sec.transtk.RU. MX
Answer
The response has no answer records.
Authority
transtk.RU. 3600 SOA dns-prim.transtk.RU. nsmaster.transtk.RU. 2024061301 900 600 86400 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0