There are a number of possible reasons: The recipient went over the number of messages their plan allowed, their phone number was typed incorrectly, they were out of cellphone range, their phone’s memory was full, or they didn’t have text messaging enabled.

If you believe this was marked in error, please go to your subscriber list and go to 'Invalid Contacts'. Find the number you are looking for, on the right you will see three dots, click on these dots and choose 'Mark As Valid'.