Question
What are spam outbound protection general recommendations?
How to prevent outbound spam?
Answer
Use SpamExperts Email Security or Plesk Email Security extensions as the most powerful tools against spam. Install ImunifyAV to detect possible malware scripts
Additional configration
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Log into Plesk.
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Remove 127.0.0.0 / 32 via the tab Plesk > Tools & Settings > Mail Server Settings > White List.
Note: Mailing Lists will be unavailable with removed localhost from White List.
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Configure Outgoing Mail Limits to prevent users from sending messages in bulk: How to set limitations on outgoing email messages in Plesk
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In case of spam is being sent using php scripts using postfix check the article Many email messages are sent from PHP scripts on a server. How to find domains on which these scripts are running if Postfix is used?
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Set more secure passwords for all mail accounts.