I've got such a file, but it's full of RedHat-specific stuff too, so it won't do you much good as is. However, the trick to doing this is fairly simple, particularly if you're using the .mc macros. Just start off with one of the typical .mc source files, remove all MAILER definitions, and add the following instead...
FEATURE(nullclient, host.to.forward.to)dnl
... where host.to.forward.to would be the fully qualified domain name of your internal mail server. The nullclient feature sets up special mailer definitions so that all mail, whether local or not, goes to the forwarding host.
According to Scott Balneaves:
Does anyone have a .mc file kicking around for sendmail that creates a forwarding only server? I'd basically like to set up an OpenBSD 3.3 firewall box running the new spamd, and a sendmail daemon that basically forwards all mail received onto our internal mail server.
I know I could write a pf rule that would forward port 25 to our internal mail server, but I like the chroot sendmail on OpenBSD, and would prefer to have it exposed to the big, bad innurnet.
Scott