public void UpdateMail_WithValidDataAndId_ShouldReturnOkObjectResult()
{
// Act
var result = sut.UpdateMail(mail, existingMailingId);
// Assert
Assert.IsInstanceOfType(result, typeof(OkObjectResult));
mockMailingRepository.Verify(m => m.Retrieve(existingMailingId), Times.Once);
mockMailingService.Verify(m => m.Save(existingMailingId, mail), Times.Once);
}