public void DeleteMail_WithMailingId_ShouldReturnNoContentResult()
{
// Act
var result = sut.DeleteMail(existingMailingId);
// Assert
Assert.IsInstanceOfType(result, typeof(NoContentResult));
mockMailingRepository.Verify(m => m.Retrieve(existingMailingId), Times.Once);
mockMailingRepository.Verify(m => m.Delete(existingMailingId), Times.Once);
}