JSONResult from MVC Controller returning HTTP 403 error

Question

I am doing a post function from a mvc view to the controller which works fine and inserts the data into the database but when i try to return the jsonresult from the controller to the view, it returns a http 403 forbidden error. i have also tried accessing that particular action directly and it's returning the same 403 forbidden error. any ideas where am i going wrong please?

Controller

 <HttpPost()> _
    Function Create(ByVal collection As ExternalMail) As JsonResult
        Try
            If ModelState.IsValid Then
                _repositoryForExternalMail.Save(collection, "")
            End If

            Return Json(New ExternalMail With {.CostCentreCode = collection.CostCentreCode, .Day = collection.Day, .Quantity = collection.Quantity, .ExternalMailItemID = collection.ExternalMailItemID, .ExternalMailLocationID = collection.ExternalMailLocationID}, JsonRequestBehavior.AllowGet)

        Catch
            Return Json("An Error Occurred")
        End Try
    End Function

JSON

$(document).ready(function () {

    var exturl = '@Url.Action("Create", "ExternalMail")';
    $("#ExtMailCreate").click(function () {


        var dataJSON = JSON.stringify(SaveExternalMail());

                    $.ajax({
                        type: "POST",
                        url: exturl,
                        data: dataJSON,
                        contentType: 'application/json; charset=utf-8',
                        success: function (data) {
                            showMessage("test", "information");
                        },
                        error: function (xhr, textStatus, errorThrown) {
                            $("#result").append(xhr.responseText);
                            $("#result").append(textStatus);
                            $("#result").append(errorThrown);
                        }


                    });

    });

});