The message can't be sent because at least one recipient isn't valid.

Hi All,

I am trying to add recipient with email id,its adding but 
when try to send email its showing 
'The message can't be sent because at least one recipient isn't valid.'Below is my JavaScript code please make me correct if wrong in code.
function PrepareActivityPartyto(partyId, partyEntityName) {
var activityParty = new Sdk.Entity("activityparty");
activityParty.addAttribute(new Sdk.String("addressused", "test43@test.com"));
activityParty.addAttribute(new Sdk.Lookup("partyid", new Sdk.EntityReference(partyEntityName, partyId)));
var activityParties = new Sdk.EntityCollection();
activityParties.addEntity(activityParty);
return activityParties;
}