At the moment, when sending messages to your customers, we can only get an acknowledgement from the carrier that the message sent was received on your customer's phone.
Unfortunately, it is not currently possible to see if the customer has read your text message or not.
There a couple of errors messages examples that you might see around the app regarding errors of message deliveries.
In the appointment section
In the automation section
In the conversation section
Directly following a message that you have sent in a conversation with your customer, a red message error might appear.
An error occurred while sending your message. Please try again.
For all previous sections, the error message means that, the phone carrier was not able to deliver the message. It could be an error in the number, a canceled number, etc.
You can also filter your messages status throughout the app to regroup the errors together and contact these customers by phone later.