This is a simple sendmail milter which adds an X-RBL-Warning header to
any emails that are received that come from an open relay as
determined by your choice of RBL checking service (i.e. bl.spamcop.net).

This is useful if you'd rather have the mail user agent (MUA) deal with
potential spam rather than just blocking it in case you loose
legitimate messages.  Note that the X-RBL-Warning header is only set if
the site was found to be an open-relay.