The handler is invoked as follows:
Handlers are not intended to be invoked by users, and they don't follow the getopt conventions.
An arbitrary string specified during the handler registration. Useful if the same executable file is being used for several handling purposes or in several modes.
The sender address. Note that the sender address is an empty string in case of bounce messages: The command line argument will be empty while still occupying a position in the argument list.
The recipient address. The order in which the recipients are listed is undefined. There is exactly one recipient address when a handler is called before local delivery, and there is one or more recipient addresses at other hooking points.