Interfacing With SpamAssassin
When spam scanning is enabled, a daemon process amavisd is conceptually very similar to a spamd process of a SpamAssassin project. In both cases forked child processes call SpamAssassin Perl modules directly, hence their performance is similar.
The main difference is in protocols used: Amavis typically speaks a standard ESMTP protocol to an MTA, while in the spamc/spamd case an MTA typically spawns a spamc program passing a message to it over a Unix pipe, then the spamc process transfers the message to a spamd daemon using a private protocol, and spamd then calls SpamAssassin Perl modules.
Read more about this topic: Amavis