What does information filtering mean by unknown sender?

I don't know who sent the letter after screening. . Filter unknown senders. This function should be known to most people. It can not only turn off the information notification sent by unknown contacts, but also classify their information separately, which will not affect the normal reception of other information, so we usually turn this function on. Open Step: Settings-Information-Filter Unknown Senders. IMessage information. Many spam messages are sent through Apple's iMessage. For example, the general SMS services of the three major operators are charged, but Apple users can send iMessage for free as long as they have a network, so if you don't use this function at ordinary times, simply and rudely turn off this function and refuse spam messages. Closing mode: Settings-Information-iMessage information. Send and receive.