Postfix version 2.3 and later support RFC 3463-style
enhanced status codes. If a command terminates with a
non-zero exit status, and the command output begins with
an enhanced status code, this status code takes precedence
over the non-zero exit status.
http://www.postfix.org/pipe.8.html说:
因此,如果您将要返回 fmilures 的地址通过管道传递给命令,然后 1) 从 RFC3463 输出增强状态代码,然后 2) 返回非零退出代码,发件人将获得您发送的增强状态代码.