настроить веб-сервер с почтовым сервером в том же домене

Это принятый ответ Кирана Шалы, адаптированный для NUnit;

var valuesController = controller;
// if your action returns: NotFound()
IHttpActionResult actionResult = valuesController.Get(10);
var notFoundRes = actionResult as NotFoundResult;
Assert.IsNotNull(notFoundRes);

// if your action returns: Ok()
actionResult = valuesController.Get(11);
var posRes = actionResult as OkResult;
Assert.IsNotNull(posRes);

// if your action was returning data in the body like: Ok<string>("data: 12")
actionResult = valuesController.Get(12);
var conNegResult = actionResult as OkNegotiatedContentResult<string>;
Assert.IsNotNull(conNegResult);
Assert.AreEqual("data: 12", conNegResult.Content);

// if your action was returning data in the body like: Content<string>(HttpStatusCode.Accepted, "some updated data");
actionResult = valuesController.Get(13);
var negResult = actionResult as NegotiatedContentResult<string>;
Assert.IsNotNull(negResult);
Assert.AreEqual(HttpStatusCode.Accepted, negResult.StatusCode);
Assert.AreEqual("some updated data", negResult.Content);