Stalker Software president and CEO Vladimir Butenko shares a few tips for protecting e-mail systems from unsolicited mail and hacker attacks.
Do limit the number of concurrent connections from a single incoming IP address -- Spammers and hackers alike will often attempt to disrupt a system by sending or bombarding the system with requests, all coming from the same IP address. This will help to prevent such intrusions.
Do limit the number of failures from a single IP -- Spammers trying to relay or to submit messages to a dictionary of addresses on a server will generate errors. If you can limit the number of failures (errors) that is acceptable before blocking the IP, this is ideal.
Do use good RBL's -- You can prevent a lot of spam from getting to your users by doing some research and figuring out which real-time blackhole lists are the most effective.
Do report known spammers -- If you discover someone is sending unsolicited e-mail, be sure to report them. This is what makes RBL's more effective.
Don't be a spam relay -- Be sure that when the mail system is configured as a closed relay. If not, you are part of the problem and shortly your server will end up blacklisted.
Don't let your users' addresses be overexposed -- prevent, to the extent you can, your users' e-mail address exposure on the Internet. Or in the very least warn them that the more their e-mail address is used on the Internet, the more likely they are to be the victims of spammers.
Don't let your users become spammers -- Monitor your user base. If they start to send unsolicited messages, your server could end up blacklisted.