The red exclamation mark (!) shows up whenever I send a picture, gif, or video, however the people I send it to can still see what I sent. This makes it hard to determine which messages actually go through and which don’t.
If I send a video, picture, or gif, occasionally the red exclamation mark will appear along with the message “Not delivered”. However, when I ask the people I sent it to if they can see the message, they tell me they can.