mail.gtnet.RU has no MX records.
Question and response
Question
dig mail.gtnet.RU. MX
Answer
The response has no answer records.
Authority
gtnet.RU. 3600 SOA inn361.RU. inn.inn361.ru.gtnet.RU. 2026020902 86400 3600 604800 345600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0