I have a proposal for a feature. If no_middleman is turned on, we
won't act as a middleman for mail. Specifically, we will only accept
mail if either the sender or the recipient is within some set of
domains. This could be used to prevent abuse of our mail gateway.
Specifically, we would not accept mail that neither originates in our
domain nor terminates in our domain (or some configurable set of
domains).
