Bouncing all mail except from known-senders

This is how I use Exim to only allow mail from known senders:

I have my first DIRECTOR like:

# 07/Mar/2002 reed
verify_known_sender:
  driver = smartuser
  require_files = /home/$local_part/.mail-accept-list
  condition = $sender_address
  senders = "! /home/$local_part/.mail-accept-list : \
            ! ^postmaster@.* : \
            ! ^mailer-daemon@.* : \
            ! ^root@.* : *"
  new_address = :fail: Sender unknown

Then the user has a simple list of email addresses (one per line) at ${HOME/.mail-accept-list. This list contains email address you will accept messages from (i.e. your friends). (This list can contain regular expressions or wildcards as defined in the Exim spec section about Address lists.)

Please note that someone could forge the sender address. Also, it allows the mailer-daemon (<>) to email so you can receive bounces.

This only bounces the email if the user has the .mail-accept-list file.

Also the fail message could be more descriptive -- maybe share a URL (using $local_part to customize).