How to Resend a Message “Not Delivered” from Mac Messages
The Mac Messages app may occasionally fail to send a message correctly, when a message fails to send it is made obvious by a little red (!) exclamation point and a “Not Delivered” message under the failed iMessage or text message. But you don’t have to retype the message or just let it go, instead you can use a little trick to resend the message again on the Mac.
This works to resend any failed message from the Messages app in Mac OS, including iMessages and text message assuming SMS has been configured to be sent and received from the Mac. Here’s how you can use this neat little trick: