You have to let users forward their email because this is
functionality they expect. The trick is to spam scan all email first,
otherwise as Alexander has said, you end up on RBL's.
Its actually a lot harder than this. Most default installations I've
seen don't take into account Return-Path notifications (i.e. passing
these notifications upstream to the origin),
What is a "default installation"?
I have a good working knowledge of all the software I have deployed in
my and my clients mail servers and I have spent a considerable amount of
time over the years furthering my understanding and perfecting my
configs.
If, by "default installation", you mean take a piece of software off the
shelf and follow a quick and dirty howto guide without any understanding
of what the options mean, then of course under these situations people
are going to run into issues.
