Лавинная рассылка трафика в порте 25

You're not actually receiving traffic on port 25, but on port 80, via your web server.

This traffic is attempting to use your server as a proxy to disguise the origin of the traffic. We generally call such servers open proxies, and they're quite useful for delivering spam and conducting attacks on other sites.

For some reason, some people seem to think your IP address has an open proxy server, but the log entries show that the requests are being refused.

If the volume of requests is too high, I would suggest firewalling off the IP address blocks rather than simply denying them in nginx. For instance:

iptables -I INPUT -s 61.231.0.0/16 -j DROP