Configure sendmail to reject mail for unknown users?

[Timothy Alberts]

I should ask a sendmail list, but this is the most responsive,
knowledgable group I've ever dealt with so...

How can I configure sendmail to reject mail for users that don't exist?
One of the annoying things that happens to my FC4 mail server all the
time is spammers/viruses will use fake accounts such as
'fakeadmin at mydomain.com' and my server always takes it because the
domain is correct.  It just gets delivered to the
postmaster at mydomain.com , I'd rather it not get into my server at all.

I think there are milters for sendmail to check against LDAP, but I
don't know the details of it yet, and I don't have an LDAP setup for the
my domain yet (still learning).  I was wondering if there is a way just
to configure sendmail to reject mail if it has a To: CC: or BCC: of a
user that doesn't really exist for a domain.