I do not know about the existance of such a smtp gateway. I think it doesn't exist.
You can control the ssl connection to your smtp server, but you have no way of controling the connections being secure or not from there to the endpoint.
In case your data is that sensitive to my opinion you never should use mail in the first place. The only way to have full control is to have your users to login to your site and show them the info they are allowed to see.