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 manual routing.

-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.