I host about 30 websites for clients on a rented server. Customers are often slow to update their Joomla and Wordpress installations. Sites get attacked quite often and I keep getting put on spam blacklists.
I would like to set up a whitelist of scripts allowed to send mail, and block all mail not coming from those scripts. The server only has 30 or so sites on it, so I can set up and manage the whitelist manually.
Has anyone heard of an existing system to do this? Any reason not to do it?