The red exclamation mark warning means that the Messages app failed to deliver the message. The issue can stem from different things depending on the type of message you're trying to send.

Your car's warning lights can let you know when something needs immediate attention, and a circle with an exclamation mark is ...

One warning light that needs immediate attention is a red circle with an exclamation mark. This symbol is the brake warning light and it's one of the most important symbols to know on your car's ...