spmfilter is a high-performance mail filtering framework, written in C.

It attempts to be a general filtering framework for any purposes.
Filtering mechanisms are provided by plugins, the API enables spmfilter
plugins to access messages as they are being processed by the MTA.
This allows them to examine and modify message content and
meta-information during the SMTP transaction. Plugins are loaded at
runtime and can be processed in any sequence, the processing chain
can also be altered by a single plugin

WWW: http://www.spmfilter.org/