spmfilter 介绍
spmfilter 是一个邮件过滤框架，其目标是成为一个通用的满足各种过滤要求的框架。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.