Workaround for mail loop for sympa-request address (#1018)
Created by: ikedas
As of Postfix 2.3, Content of Return-Path: pseudo-header field added by pipe(8) for null envelope sender became "MAILER-DAEMON". Empty null_sender
option will prevent this behavior.
As a workaround, the uppercase "" will be treated as null envelope sender.
This is a workaround for #1018 (closed) .