I stumbled across this by accident the other day. I was exchanging messages with a friend, and the last message I sent would not go. We were exchanging messages using iMessage. As the last one was not going I assumed that he did not have a data connection, which was why he couldn't get the message.
When I pressed to retry I must of long pressed, not intentional and instead of it saying copy it said "Send as Text Message".
This will only work when the iMessage can't be delivered, so if it is important and you can't wait for them to be in data signal you could try it as an SMS.
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