Apple's mobile phone information has a red exclamation mark. What is the red exclamation mark on Apple's mobile phone information?

The message 1. has a red exclamation mark because you failed to send the message.

2. If you find this email, please open it to show that it failed to send. The message is undelivered with a red exclamation mark. Click the exclamation point and select Try sending the message again. If you still can't send the message, please open the phone settings, find the message, and open iMessage before sending.