preline - prepend lines to message
in .qmailext: | preline command
preline feeds each incoming mail message through command.
At the top of each message it inserts a UUCP-style From_
line, a Return-Path line, and a Delivered-To line.
preline is useful for procmail and ELM's filter, which do
not understand the qmail-command environment variables.
-d Do not include the Delivered-To line. You should
use this option when the recipient of the incoming
mail message is actually under remote control, but
was sent here through control/virtualdomains for
-f Do not include the From_ line. You should use this
option except for commands that create mbox files.
-r Do not include the Return-Path line.