Pipe
From Mailutils
Pipe is a Sieve extension action.
Syntax
pipe [:envelope] [:header] [:body] <program: string>
The pipe action executes a shell command specified by its argument and pipes the entire message, including its envelope, to its standard input.
Piping only certain parts of the message can be requested by supplying corresponding tagged arguments. For example
pipe :envelope :body "prog"
will send to prog only envelope and body of each message.