The 1913 Webster provides a different, much more negative definition:
Emissary \Em"is*sa*ry\, n.; pl. {Emissaries}. [L. emissarius,
fr. emittere, emissum, to send out: cf. F. missaire. See
{Emit}.]
An agent employed to advance, in a covert manner, the
interests of his employers; one sent out by any power that is
at war with another, to create dissatisfaction among the
people of the latter.
Buzzing emissaries fill the ears
Of listening crowds with jealousies and fears.
--Dryden.
Syn: {Emissary}, {Spy}.
Usage: A spy is one who enters an enemy's camp or territories
to learn the condition of the enemy; an emissary may
be a secret agent appointed not only to detect the
schemes of an opposing party, but to influence their
councils. A spy must be concealed, or he suffers
death; an emissary may in some cases be known as the
agent of an adversary without incurring similar
hazard.
I don't think this merely subtleties in defining, as w:emissary goes to ambassadors and diplomats. Is there a serious change in usage over time that should be noted here?--Prosfilaes 22:06, 25 October 2010 (UTC)