Why is there an explanation mark on my iMessage icon

Estimate how long that that exclamation mark has been there. Then determine all the contacts that you may have sent messages to during that time. Open each contact window and scroll down a few texts and see whether you see a sent message marked with the same exclamation mark. This may take time however can be fast if the exclamation point was recent.

Axel F.

The Messages app is notifying you that one or more messages have not been sent successfully.

Axel F.