ERROR: I could not complete a connection to any of your mailservers!

istud.ru: Timed out [Last data sent: RCPT TO: ]

If this is a timeout problem, note that the DNS report only waits about 40 seconds for responses, so your mail *may* work fine in this case but you will need to use testing tools specifically designed for such situations to be certain.

COMMAND   PID        USER   FD   TYPE DEVICE SIZE NODE NAME
exim4    2236 Debian-exim    4u  IPv4   5015       TCP *:smtp (LISTEN)
exim4   27551 Debian-exim    5u  IPv4 182475       TCP 85.249.72.106:smtp->test.DNSstuff.com:39507 (ESTABLISHED)
exim4   27551 Debian-exim    6u  IPv4 182475       TCP 85.249.72.106:smtp->test.DNSstuff.com:39507 (ESTABLISHED)
lsof -i :25
COMMAND   PID        USER   FD   TYPE DEVICE SIZE NODE NAME
exim4    2236 Debian-exim    4u  IPv4   5015       TCP *:smtp (LISTEN)
exim4   27551 Debian-exim    5u  IPv4 182475       TCP 85.249.72.106:smtp->test.DNSstuff.com:39507 (ESTABLISHED)
exim4   27551 Debian-exim    6u  IPv4 182475       TCP 85.249.72.106:smtp->test.DNSstuff.com:39507 (ESTABLISHED)
lsof -i :25
COMMAND   PID        USER   FD   TYPE DEVICE SIZE NODE NAME
exim4    2236 Debian-exim    4u  IPv4   5015       TCP *:smtp (LISTEN)
exim4   27551 Debian-exim    5u  IPv4 182475       TCP 85.249.72.106:smtp->test.DNSstuff.com:39507 (ESTABLISHED)
exim4   27551 Debian-exim    6u  IPv4 182475       TCP 85.249.72.106:smtp->test.DNSstuff.com:39507 (ESTABLISHED)
 lsof -i :25
COMMAND   PID        USER   FD   TYPE DEVICE SIZE NODE NAME
exim4    2236 Debian-exim    4u  IPv4   5015       TCP *:smtp (LISTEN)
exim4   27551 Debian-exim    5u  IPv4 182475       TCP 85.249.72.106:smtp->test.DNSstuff.com:39507 (CLOSE_WAIT)
exim4   27551 Debian-exim    6u  IPv4 182475       TCP 85.249.72.106:smtp->test.DNSstuff.com:39507 (CLOSE_WAIT)