This is not iOS 11 specific but since most peeps are on 11 I'll ask here. This has made me crazy for years. You send a text (iMessage) with some pics, maybe a video, and the files are pretty big and the text is "sending" for really long long periods of time. During this time you basically can't message anyone else. It just clogs up iMessage waiting for the original message to go out. You can delete the specific messages, shut down iMessage, turn off all data connections, etc, You can even reboot the phone after deleting the message and the damn thing just keeps trying to send! It never quits. Am I missing some secret way to cancel an outgoing message?