What does it mean to add an exclamation point to Apple's SMS?

There is an exclamation mark on the SMS icon of iPhone, indicating that there is an unsuccessful message to send in the SMS. It can be solved in the following ways:

1. First, we turn on the phone and click the "Settings" system icon.

2. Then we click the "Information" option, as shown below.

3. Then we open the "iMessage" option on the right, close it and restart it.

But to clear the exclamation mark on the message, we need to turn off the Apple mobile phone.

5. Then turn on the Apple phone later. At this point, click the "Information" icon again.

6. Then send the unsent message.