The handler is invoked as follows:

# ./handler context sender recipient [recipient]...[recipient]

Handlers are not intended to be invoked by users, and they don’t follow
the getopt conventions.

Arguments

context

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.

sender

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.

recipient

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.