On Mon, 26 Apr 1999, Tom Trebisky wrote:

>  As near as I can tell this means I am running sendmail 8.8.8.4
>  with a default sendmail.cf.
>  Two different parties have pointed out to me that my machine is
>  an open mail relay, which as most anyone knows is a BAD thing in
>  a world that would sooner be without spam.

The hosts that you allow relaying from are listed in the Cw variable. 
AFAIK that is only `localhost' by default. `grep ^Cw /etc/sendmail.cf'
should return only "Cwlocalhost". If you have something really nasty
there, like a lone `.', take it out.

All the custom solutions floating around are for the benefit of remote
users. If you don't have any of those, just disable relaying with "Cw".