When the system detects an error in the phone number or the email it will notify you and disable the option until it is fixed.
There are different errors you may have when sending SMS texts.
The most common would be to have an incorrect phone number, you'll get a message in the client's activity feed.
There are some other possible messages you may get, in case this happens you can go to https://www.twilio.com/docs/api/errors and check the error according to the message you get in the activity feed.
Regarding the email errors, there are some possible errors like the email address is not correct or it bounced. In these cases, you can check on the information you have and update the correct email address or contact the client by SMS to ask them to check their email account.
If you get this message and you clients say they did not notify us to stop emailing them, please ask your client to send an email to the support team to help them out.