by DigitalOcean
About Spamassassin
Spamassassin is a free and open-source mail filter written in Perl that is used to identify spam using a wide range of heuristic tests on mail headers and body text. It will save your mailbox from much unwanted spam emails.
Prerequisites
Before installing Spamassassin, you need to install and setup a mail transfer agent such as Postfix on your virtual private server.
You can find instructions on that here