Well, first, the enemy emissary would still have to be present for you to get the offer/proposal. I don't believe that you get more messages, but you can use them as agents to see everything that's going on in that province. Until the enemy assassinates them, that is, and boosts his own assassins this way.
Spies can reveal whether something's cooking in your own provinces, such as rebellion or civil war, but I don't know whether they can do anything else in enemy provinces apart from informing you that the enemy is preparing to launch a major attack on somebody/you as far as the messages are concerned.