>*please* tell me it's not a Solaris so old that it still has the @@ bug
>in expanding a null MAIL FROM:<> into a Return-Path? ;)
>
>If so, there's a patch at ftp://ftp.lsoft.com/listserv/unix/contrib

If this is the case, a better fix would be to upgrade sendmail.  If I
recall correctly, that patch is for sendmail 8.6, which is about as insecure
from spam as you can get.

Nathan