i'm currently trying to improve our email setup in sense of spam
rejecting and therefore trying to setup a kind of Sender Verification
where Mails are rejected on SMTP-time, when all of the following
conditions are met:
* Sender Address is an e-mail address which is, by means of the domain,
considered to be local
* Sender Host is not an authorized mailserver (e.g. there is a hostlist
which holds each server which is allowed to send in emails with our
local-addresses)
