Answer:

The red exclamation mark on WeChat's message to the other side means that the message failed to be sent, please resend. You can click on the red exclamation mark to resend it.

The red exclamation mark in a chat message means that the message failed to be sent due to the network, so please resend it later.

This is a long time coming, so please change the network access point of your cell phone (e.g., Wifi) and log in to WeChat again.

The person who added you to the blacklist can't receive the message you sent, so there will be a red exclamation mark, and if you resend the message, there will still be an exclamation mark, so you need to add friends to re-verify the message.