smtp.rbc.ru has no MX records.
Question and response
Question
dig smtp.rbc.ru. MX
Answer
The response has no answer records.
Authority
rbc.ru. 60 SOA ns.rbc.ru. noc.rbc.ru. 2026022403 3600 1200 604800 60
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0