spmfilter - mail filtering framework
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, for example if the plugin has to stop further processing (e.g. the clamav-plugin detected malicious software like a virus and the infected message is actually discarded - so further processing is stopped by the plugin).