Activate Reverse DNS to block bogus senders This could be very helpful to mitigate spam floods and DoS attacks that target your network infrastructure. To maintain optimal values for these parameters may require refinement over time. The main parameters used to handle connection limits include: total number of connections, total number of simultaneous connections, and maximum connection rate. These parameters depend on the specifications of the server hardware (memory, NIC bandwidth, CPU, etc.) and its nominal load per day. The number of connections to your SMTP server should be limited.

Limit connections to protect your server against DoS attacks This configuration is highly recommended when your mail server has a routed IP address. If configured the right way, only known accounts can use your servers SMTP to send email. This helps to prevent open relay and abuse of your server. SMTP Authentication forces the people who use your server to obtain permission to send mail by first supplying a username and password. Set up SMTP authentication to control user access Misconfiguration of this option can harm you because spammers can use your mail server (and network resources) as a gateway for spamming others, resulting in your getting blacklisted. In other words, this parameter specifies for whom your SMTP protocol should forward mail. All mail servers have this option, where you can specify which domains or IP addresses your mail server will relay mail for. It’s very important to configure your mail relay parameter to be very restrictive.

Configure mail relay options carefully to avoid being an Open Relay

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