A method for filtering spam directed at mobile phones in a short message system (SMS). A short message includes message content, and both a mobile phone number and a first authentication code of a destination mobile phone. A second authentication code of the destination mobile phone is stored in a memory. The short message is received and the first authentication code and the second authentication code are extracted from the short message and the memory, respectively. If it is ascertained that the extracted first authentication code does not match the extracted second authentication code, then the short message is determined to be SMS spam and is prevented from being subsequently received by the destination mobile phone. Otherwise the short message is determined to not be SMS spam and is not prevented from being subsequently received by the destination mobile phone. An authentication apparatus within the SMS performs the method.