Allowing email only from certain domains/addresses?

Hello everyone,

I need to configure Sendmail to allow email from only certain domains
and email addresses (two domains and one additional email address, to
be specific). I'm not familiar with doing special configuration on
Sendmail at all... could someone point me in the right direction?

It seems like there should be a simple way to say "block everything
but this, this, and this", but doing a few quick Google searches I
haven't been able to find anything.

Thanks!
Jacob