What are spam outbound protection recommendations on a Plesk for Linux server?

Question

What are the general recommendations for outbound spam protection? 
How to prevent outbound spam?

Answer

The most effective way to protect the server from outbound spam is to use specialized extensions:

• Plesk Email Security (Free or Pro) or SpamExperts Email Security.
• ImunifyAV to detect and clean malware scripts that may send spam.

Additional Configuration

1. Log in to Plesk.

2. Remove the localhost network (127.0.0.0/8) from the White List to force authentication for local scripts:

   Go to Tools & Settings Mail Server Settings White List tab. Select the 127.0.0.0/8 network and click Remove.

   

   Note: Mailing Lists will be unavailable if the localhost network is removed from the White List.

3. Configure Outgoing Mail Limits to prevent users from sending messages in bulk. See the following article for details: 
   How to set limitations on outgoing email messages in Plesk
4. If spam is being sent via PHP scripts using Postfix, check this article to identify the source: 
   Many email messages are sent from PHP scripts on a server. How to find domains on which these scripts are running?
5. Set secure (strong) passwords for all mail accounts to prevent brute-force attacks.