I'm currently in a position where there are two of us, and neither of us should be able to send an email from a joint account without the explicit approval of both of us. Ideally, I'm looking for software which will basically allow either of us to compose an email, and then hold it until both of us have clicked a "Send" button. The obvious generic extension of this which has a slightly likelier chance of existing would require m approvals from a total of n people.
In particular, we are looking for an automated solution to ensure that neither of us accidentally thinks that the other has given implicit approval (say verbally) when in fact that hasn't happened. The attack vector of one of us forging an SMTP header is not one that we are trying to defeat.