A handler is an executable file which is invoked at the selected points (called hooking points).Mail handlers are used for handling incoming, outgoing, and pass-through mail at various stages of processing by the MTA. One or more handlers can be applied to a message.
Note: Every handler you use should be registered by the mailmng utility. For details, refer to the Using mailmng Utility section.
For implementation details, refer to the Using Mail Handlers with qMail section of the Developer's Guide.