Register a new SMTP filter in the filter chain.
Specifies where in the filter chain this new filter will reside
|SMTP_FILTER_PRIORITY_CLASS_SOURCE||For filters that need to see messages as soon as they leave the source|
|SMTP_FILTER_PRIORITY_CLASS_CACHE||For filters that provide cache like functionality|
|SMTP_FILTER_PRIORITY_CLASS_NORMAL||For normal filters like scanners and categorisers|
|SMTP_FILTER_PRIORITY_CLASS_EDGE||For filters that need to see messages and data just before it goes out|
The filter might not actually be inserted at this priority if a filter already exists there. In this case the next highest available priority is chosen. The return value from this function informs you of the actual priority.
Pointer to a SMTPFilterProviderInterface for the framework to use to interface with your filter