What are spam outbound protection general recommendations?
How to prevent outbound spam?
This is the recommended solution by Plesk and here are the instructions on how to install it.
Additional manual actions
Log into Plesk.
Remove 127.0.0.0 / 32 via the tab Plesk > Tools & Settings > Mail Server Settings > White List.
Configure Outgoing Mail Limits to prevent users from sending messages in bulk: How to set limitations on outgoing email messages in Plesk
- 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?
- Set more secure passwords for all mail accounts.
- For Linux, as another option, you can take a look at MailChannels extension, it should filter the outgoing mail on their servers
Additionally, the following can be checked: Protection from Outbound Spam