Spam connections increase

Lately, I got a lot of connections on my SMTP server. This is the primary system used to receive and send emails.

The SMTP system has been flooded by connections. Our system limits those connections to a small number to avoid overflows of the system as a whole. This means if the maximum number is reached you may be prevented from connecting since the maximum number of connections is reached pretty much permanently.

What I have done since I noticed the problem is add some IP addresses in my firewall to completely block the worst offenders. This works great since they cannot connect at all when they are blocked in that manner. Preventing them from flooding our system.

The worst I've got so far, after I blocked it, sent us nearly 22,000 requests for a whooping 1.25Mb. Here you can see the firewall information:

21952  1317381 REJECT   tcp  --  *   *      tcp dpt:25

Even the HTTP requests that our system automatically block aren't that bad. Most of the worst only reach a mere 1,200 or so.