How do I configure sendmail to disallow relaying mail that is neither to
nor from a machine in my domain?

I've got a little network hidden behind a masquerading firewall, under
the domain `manumission.org'. All my machines are configured to relay
all  mail through the firewall machine, and the firewall is configured
to rewrite the envelope to strip machine names to just
`user@manumission.org'. But this still allows `spammer@spam.net' to
relay mail through my machine, which I'd like to disallow.  Do I disable
the `smtp' mailer?

I know this isn't a NetBSD question (and the firewall in question is a
linux box), but you're some of the most knowledgable people I know. I've
read the sendmail FAQ, and it hasn't helped. Appended is my .mc file.

VERSIONID(`cellwall.mc')
OSTYPE(linux)dnl
DOMAIN(manumission.org)dnl
define(`PROCMAIL_MAILER_PATH',`/usr/bin/procmail')dnl
FEATURE(`smrsh',`/usr/sbin/smrsh')dnl
FEATURE(use_cw_file)dnl
FEATURE(nouucp)dnl
FEATURE(local_procmail)
MAILER(procmail)
MAILER(smtp)

Thanks, and happy new year!