To protect your users from spam, you can use the following tools with your control panel:
You can configure the spam filter so as to either delete suspicious messages when they come to your mail server, or change the subject line and add "X-Spam-Flag: YES" and "X-Spam-Status: Yes" headers to the messages. The latter can be useful for users who prefer to filter mail with mail filtering programs installed on their own computers.
To learn more about SpamAssassin, visit http://spamassassin.apache.org.
To configure and switch on the SpamAssassin filter, proceed to the Setting Up SpamAssassin Spam Filter section.
To configure your mail server for working with DNSBL databases, proceed to the Switching On Spam Protection Based on DNS Blackhole Lists section.
To set up server-wide black and white lists, proceed to the Setting Up Server-wide Black and White Lists section.